Integrated power generation and carbon capture using fuel cells

ABSTRACT

Systems and methods are provided for capturing CO 2  from a combustion source using molten carbonate fuel cells (MCFCs). The fuel cells are operated to have a reduced anode fuel utilization. Optionally, at least a portion of the anode exhaust is recycled for use as a fuel for the combustion source. Optionally, a second portion of the anode exhaust is recycled for use as part of an anode input stream. This can allow for a reduction in the amount of fuel cell area required for separating CO 2  from the combustion source exhaust and/or modifications in how the fuel cells are operated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Ser. Nos. 61/787,587,61/787,697, 61/787,879, and 61/788,628, all filed on Mar. 15, 2013, eachof which is incorporated by reference herein in its entirety. Thisapplication also claims the benefit of U.S. Ser. Nos. 61/884,376,61/884,545, 61/884,565, 61/884,586, 61/884,605, and 61/884,635, allfiled on Sep. 30, 2013, each of which is incorporated by referenceherein in its entirety. This application further claims the benefit ofU.S. Ser. No. 61/889,757, filed on Oct. 11, 2013, which is incorporatedby reference herein in its entirety.

This application is related to 25 other co-pending U.S. applications,filed on even date herewith, and identified by the following AttorneyDocket numbers and titles: 2013EM104-US2 entitled “Integrated PowerGeneration and Carbon Capture using Fuel Cells”; 2013EM104-US3 entitled“Integrated Power Generation and Carbon Capture using Fuel Cells”;2013EM107-US2 entitled “Integrated Power Generation and Carbon Captureusing Fuel Cells”; 2013EM108-US2 entitled “Integrated Power Generationand Carbon Capture using Fuel Cells”; 2013EM108-US3 entitled “IntegratedPower Generation and Carbon Capture using Fuel Cells”; 2013EM109-US2entitled “Integrated Power Generation and Carbon Capture using FuelCells”; 2013EM109-US3 entitled “Integrated Power Generation and CarbonCapture using Fuel Cells”; 2013EM272-US2 entitled “Integrated PowerGeneration and Chemical Production using Fuel Cells”; 2013EM273-US2entitled “Integrated Power Generation and Chemical Production using FuelCells at a Reduced Electrical Efficiency”; 2013EM274-US2 entitled“Integrated Power Generation and Chemical Production using Fuel Cells”;2013EM277-US2 entitled “Integrated Power Generation and ChemicalProduction using Fuel Cells”; 2013EM278-US2 entitled “Integrated CarbonCapture and Chemical Production using Fuel Cells”; 2013EM279-US2entitled “Integrated Power Generation and Chemical Production using FuelCells”; 2013EM285-US2 entitled “Integrated Operation of Molten CarbonateFuel Cells”; 2014EM047-US entitled “Mitigation of NOx in IntegratedPower Production”; 2014EM048-US entitled “Integrated Power Generationusing Molten Carbonate Fuel Cells”; 2014EM049-US entitled “Integrated ofMolten Carbonate Fuel Cells in Fischer-Tropsch Synthesis”; 2014EM050-USentitled “Integrated of Molten Carbonate Fuel Cells in Fischer-TropschSynthesis”; 2014EM051-US entitled “Integrated of Molten Carbonate FuelCells in Fischer-Tropsch Synthesis”; 2014EM052-US entitled “Integratedof Molten Carbonate Fuel Cells in Methanol Synthesis”; 2014EM053-USentitled “Integrated of Molten Carbonate Fuel Cells in a RefinerySetting”; 2014EM054-US entitled “Integrated of Molten Carbonate FuelCells for Synthesis of Nitrogen Compounds”; 2014EM055-US entitled“Integrated of Molten Carbonate Fuel Cells with Fermentation Processes”;2014EM056-US entitled “Integrated of Molten Carbonate Fuel Cells in Ironand Steel Processing”; and 2014EM057-US entitled “Integrated of MoltenCarbonate Fuel Cells in Cement Processing”. Each of these co-pendingU.S. applications is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

In various aspects, the invention is related to low emission powerproduction with separation and/or capture of resulting emissions viaintegration of molten carbonate fuel cells with a combustion powersource.

BACKGROUND OF THE INVENTION

Capture of gases emitted from power plants is an area of increasinginterest. Power plants based on the combustion of fossil fuels (such aspetroleum, natural gas, or coal) generate carbon dioxide as a by-productof the reaction. Historically this carbon dioxide has been released intothe atmosphere after combustion. However, it is becoming increasinglydesirable to identify ways to find alternative uses for the carbondioxide generated during combustion.

One option for managing the carbon dioxide generated from a combustionreaction is to use a capture process to separate the CO₂ from the othergases in the combustion exhaust. An example of a traditional method forcapturing carbon is passing the exhaust stream through an aminescrubber. While an amine scrubber can be effective for separating CO₂from an exhaust stream, there are several disadvantages. In particular,energy is required to operate the amine scrubber and/or modify thetemperature and pressure of the exhaust stream to be suitable forpassing through an amine scrubber. The energy required for CO₂separation reduces the overall efficiency of the power generationprocess.

In order to offset the power required for CO₂ capture, one option is touse a molten carbonate fuel cell to assist in CO₂ separation. The fuelcell reactions that cause transport of CO₂ from the cathode portion ofthe fuel cell to the anode portion of the fuel cell can also result ingeneration of electricity. However, conventional combinations of acombustion powered turbine or generator with fuel cells for carbonseparation have resulted in a net reduction in power generationefficiency per unit of fuel consumed.

An article in the Journal of Fuel Cell Science and Technology (G.Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol. 9, February 2012)describes a power generation system that combines a combustion powergenerator with molten carbonate fuel cells. Various arrangements of fuelcells and operating parameters are described. The combustion output fromthe combustion generator is used in part as the input for the cathode ofthe fuel cell. This input is supplemented with a recycled portion of theanode output after passing through the anode output through a cryogenicCO₂ separator.

One goal of the simulations in the Manzolini article is to use the MCFCto separate CO₂ from the power generator's exhaust. The simulationdescribed in the Manzolini article establishes a maximum outlettemperature of 660° C. and notes that the inlet temperature must besufficiently cooler to account for the temperature increase across thefuel cell. The electrical efficiency (i.e. electricity generated/fuelinput) for the MCFC fuel cell in a base model case is 50%. Theelectrical efficiency in a test model case, which is optimized for CO₂sequestration, is also 50%.

An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37,2012) describes a method for modeling the performance of a powergeneration system using a fuel cell for CO₂ separation. Recirculation ofanode exhaust to the anode inlet and the cathode exhaust to the cathodeinlet are used to improve the performance of the fuel cell. Based on themodel and configuration shown in the article, increasing the CO₂utilization within the fuel cell is shown as being desirable forimproving separation of CO₂. The model parameters describe an MCFCelectrical efficiency of 50.3%.

U.S. Pat. No. 7,396,603 describes an integrated fossil fuel power plantand fuel cell system with CO₂ emissions abatement. At least a portion ofthe anode output is recycled to the anode input after removal of aportion of CO₂ from the anode output.

Molten carbonate fuel cells utilize hydrogen and/or other fuels togenerate electricity. The hydrogen may be provided by reforming methaneor other reformable fuels in a steam reformer that is upstream of thefuel cell or within the fuel cell. Reformable fuels can encompasshydrocarbonaceous materials that can be reacted with steam and/or oxygenat elevated temperature and/or pressure to produce a gaseous productthat comprises hydrogen. In particular, reformable fuel can include, butis not limited to, alkanes, alkenes, alcohols, aromatics, and/or othercarbonaceous and organic compounds that can be reformed to generate H₂and carbon oxides (either CO or CO₂). Alternatively or additionally,fuel can be reformed in the anode cell in a molten carbonate fuel cell,which can be operated to create conditions that are suitable forreforming fuels in the anode. Alternately or additionally, the reformingcan occur both externally and internally to the fuel cell.

Traditionally, molten carbonate fuel cells are operated to maximizeelectricity production per unit of fuel input, which may be referred toas the fuel cell's electrical efficiency. This maximization can be basedon the fuel cell alone or in conjunction with another power generationsystem. In order to achieve increased electrical production and tomanage the heat generation, fuel utilization within a fuel cell istypically maintained at 70% to 75%.

U.S. Published Patent Application 2011/0111315 describes a system andprocess for operating fuel cell systems with substantial hydrogencontent in the anode inlet stream. The technology in the '315publication is concerned with providing enough fuel in the anode inletso that sufficient fuel remains for the oxidation reaction as the fuelapproaches the anode exit. To ensure adequate fuel, the '315 publicationprovides fuel with a high concentration of H₂. The H₂ not utilized inthe oxidation reaction is recycled to the anode for use in the nextpass. On a single pass basis, the H₂ utilization may range from 10% to30%. The '315 reference does not describe significant reforming withinthe anode, instead relying primarily on external reforming.

U.S. Published Patent Application 2005/0123810 describes a system andmethod for co-production of hydrogen and electrical energy. Theco-production system comprises a fuel cell and a separation unit, whichis configured to receive the anode exhaust stream and separate hydrogen.A portion of the anode exhaust is also recycled to the anode inlet. Theoperating ranges given in the '810 publication appear to be based on asolid oxide fuel cell. Molten carbonate fuel cells are described as analternative.

U.S. Published Patent Application 2003/0008183 describes a system andmethod for co-production of hydrogen and electrical power. A fuel cellis mentioned as a general type of chemical converter for converting ahydrocarbon-type fuel to hydrogen. The fuel cell system also includes anexternal reformer and a high temperature fuel cell. An embodiment of thefuel cell system is described that has an electrical efficiency of about45% and a chemical production rate of about 25% resulting in a systemcoproduction efficiency of about 70%. The '183 publication does a) notappear to describe the electrical efficiency of the fuel cell inisolation from the system.

U.S. Pat. No. 5,084,362 describes a system for integrating a fuel cellwith a gasification system so that coal gas can be used as a fuel sourcefor the anode of the fuel cell. Hydrogen generated by the fuel cell isused as an input for a gasifier that is used to generate methane from acoal gas (or other coal) input. The methane from the gasifier is thenused as at least part of the input fuel to the fuel cell. Thus, at leasta portion of the hydrogen generated by the fuel cell is indirectlyrecycled to the fuel cell anode inlet in the form of the methanegenerated by the gasifier.

SUMMARY OF THE INVENTION

In one aspect, a method for capturing carbon dioxide from a combustionsource is provided. The method can introducing one or more fuel streamsand an O₂-containing stream into a combustion zone; performing acombustion reaction in the combustion zone to generate a combustionexhaust, the combustion exhaust comprising CO₂; processing at least afirst portion of the combustion exhaust with a fuel cell array of one ormore molten carbonate fuel cells to form a cathode exhaust stream fromat least one cathode outlet of the fuel cell array, the one or more fuelcells each having an anode and a cathode, the molten carbonate fuelcells being operatively connected to the combustion exhaust through oneor more cathode inlets in the fuel cell array; reacting carbonate fromthe one or more fuel cell cathodes with hydrogen within the one or morefuel cell anodes to produce electricity, an anode exhaust stream from atleast one anode outlet of the fuel cell array comprising CO₂ and H₂;separating CO₂ from the anode exhaust stream in one or more separationstages to form a CO₂-depleted anode exhaust stream; passing at least afirst portion of the CO₂-depleted anode exhaust stream to the combustionzone; and recycling at least a second portion of the CO₂-depleted anodeexhaust stream to one or more of the fuel cell anodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 2 schematically shows an example of the operation of a moltencarbonate fuel cell.

FIG. 3 shows an example of the relation between anode fuel utilizationand voltage for a molten carbonate fuel cell.

FIG. 4 schematically shows an example of a configuration for an anoderecycle loop.

FIG. 5 shows an example of the relation between CO₂ utilization,voltage, and power for a molten carbonate fuel cell.

FIG. 6 schematically shows an example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 7 schematically shows another example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 8 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 9 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIGS. 10-15 show results from simulations of various configurations of apower generation system including a combustion-powered turbine and amolten carbonate fuel cell for carbon dioxide separation.

FIGS. 16 and 17 show examples of CH₄ conversion at different fuel celloperating voltages V_(A).

DETAILED DESCRIPTION OF THE EMBODIMENTS

In various aspects, systems and methods are provided for capturing CO₂from a combustion source using molten carbonate fuel cells (MCFCs). Thesystems and methods can address one or more problems related to carboncapture from combustion exhaust stream and/or performing carbon captureusing molten carbonate fuel cells.

One difficulty with conventional uses of carbon capture technology inconjunction with a combustion-based power source for power generation,such as use of molten carbonate fuel cells as part of a carbon capturescheme, is that the overall efficiency of the power generation system isreduced. Although molten carbonate fuel cells can generate electricalpower, so that the net power generated by a system is increased,conventional combinations of fuel cells with combustion-poweredgenerators result in net lower power efficiency for the power plant as awhole. In other words, the electrical power produced (watts) per unit offuel input (lower heating value of the fuel, kJ) is reduced. This can bedue in part to additional power or heating requirements for operatingthe additional carbon capture components. This can also be due in partto a lower efficiency of power generation for conventionally operatedfuel cells in comparison with a system such as a combustion-poweredturbine.

In some aspects, the overall efficiency of a carbon capture system thatincludes molten carbonate fuel cells can be improved by operating thefuel cells at lower anode fuel utilization values. Conventionally,molten carbonate fuel cells can be operated at a fuel utilization thatbalances the heat needed to operate the fuel cell with the fuel consumedwithin the cell. The fuel utilization in conventional fuel cells cantypically be made as high as possible while maintaining this heatbalance. By contrast, it has been determined that, for various types ofpower system configurations, reducing the anode fuel utilization of fuelcell array can allow for improved power generation efficiency for theoverall system.

Another difficulty with using molten carbonate fuel cells for separationof CO₂ from an exhaust stream can include the large area of fuel cellstypically required for handling the exhaust from a commercial scaleturbine or other power/heat generator. Accommodating a commercial scaleexhaust flow using molten carbonate fuel cells can typically involveusing a plurality of fuel cells, rather than constructing a single fuelcell of sufficient area. In order to deliver the exhaust stream to thisplurality of fuel cells, additional connections can be required in orderto divide the exhaust between the various fuel cells. Thus, reducing thefuel cell area required to capture a desired amount of carbon dioxidecan provide a corresponding decrease in the number and/or complexity offlow connections required.

In some aspects of the invention, the area of fuel cells required forprocessing a CO₂-containing exhaust stream can be reduced or minimizedby recycling at least a portion of the anode exhaust stream back to theanode inlet. Additionally or alternately, the fuel cells can be operatedat lower fuel utilization. An exhaust stream can be passed into thecathode(s) of molten carbonate fuel cells. During operation of the fuelcell, the anode exhaust can be passed through one or more separationstages. This can include separation stages for removal of H₂O and/orCO₂. At least a portion of the remaining anode exhaust can then berecycled to the anode input. In one preferred embodiment, any recycle ofthe anode exhaust directly to the cathode can be avoided. By recyclingat least some portion the anode exhaust to the anode inlet, at leastsome of the fuel not used on the first pass through the anode can beutilized in a subsequent pass.

In addition to or as an alternative to recycling the anode exhaust tothe anode inlet, at least a portion of the hydrogen in the anode exhaustcan be recycled to the combustion zone for a turbine or othercombustion-powered generator/heat source. It is noted that any hydrogengenerated via reforming as part of the anode loop can represent a fuelwhere the CO₂ has already been “captured” by transfer to the anode loop.This can reduce the amount of CO₂ needing to be transferred from thecathode side to the anode side of the fuel cell, and therefore can leadto a reduced fuel cell area.

An additional or alternative feature that can contribute to a reducedfuel cell area can be reducing and/or minimizing the amount of energyrequired for processes not directly involved in power generation. Forexample, the anode reaction in a molten carbonate fuel cell can combineH₂ with CO₃ ²⁻ ions transported across the electrolyte between cathodeand anode to form H₂O and CO₂. Although the anode reaction environmentcan facilitate some reforming of a fuel such as CH₄ to form H₂, some H₂can advantageously be present in a fuel in order to maintain desirablereaction rates in the anode. As a result, prior to entering the anodeitself, fuels (such as natural gas/methane) are conventionally at leastpartially reformed prior to entering the anode. The reforming stageprior to the anode for a fuel cell can require additional heat in orderto maintain a suitable temperature for reforming.

In some aspects of the invention, recycling at least a portion of theanode exhaust to the anode inlet can allow for a reduced amount ofreforming and/or elimination of the reforming stage prior to the anodeinlet. Instead of reforming a fuel stream prior to entering the anode,the recycled anode exhaust can provide sufficient hydrogen for the fuelinput to the anode. This can allow the input stream for the anode to bepassed into the anode without passing through a separate pre-reformingstage. Operating the anode at a reduced level of hydrogen fuelutilization can further facilitate reducing and/or eliminating thepre-reforming stage by providing an anode exhaust with increasedhydrogen content. Increasing the hydrogen content can allow a portion ofthe anode exhaust to also be used as an input to the turbine combustionzone, while still having sufficient hydrogen in the feed to the anodeinlet so that pre-reforming can be reduced and/or eliminated.

Another challenge with using molten carbonate fuel cells can be due tothe relatively low CO₂ content of the exhaust of properly operated gasturbine. For example, a gas turbine powered by a low CO₂ content naturalgas fuel source can generate an exhaust, for example, with a CO₂ ofabout 4 vol %. If some type of exhaust gas recycle is used, this valuecan be raised, for example, to about 6 vol %. By contrast, a typicaldesired CO₂ content for the input to the cathode of a molten carbonatefuel cell can be about 10% or more. In some aspects of the invention,systems and methods are provided herein to allow for increased CO₂content in the exhaust gas while still efficiently operating the gasturbine or other combustion powered generator. In some aspects of theinvention, systems and methods are provided for improving and/oroptimizing the efficiency of carbon capture by the fuel cell whenoperated with a cathode exhaust having a low CO₂ content.

Still another challenge can include reducing or mitigating the loss ofefficiency in power generation caused by carbon capture. As noted above,conventional methods of carbon capture can result in a loss of netefficiency in power generation per unit of fuel consumed. In someaspects of the invention, systems and methods are provided for improvingthe overall power generation efficiency. Additionally or alternately, insome aspects of the invention, methods are provided for separating CO₂in a manner to reduce and/or minimize the energy required for generationof a commercially valuable CO₂ stream.

In most aspects of the invention, one or more of the above advantagescan be achieved, at least in part, by using molten carbonate fuel cellsin combination with a combined cycle power generation system, such as anatural gas fired combined cycle plant, where the flue gas and/or heatfrom combustion reaction(s) can also be used to power a steam turbine.More generally, the molten carbonate fuel cells can be used inconjunction with various types of power or heat generation systems, suchas boilers, combustors, catalytic oxidizers, and/or other types ofcombustion powered generators. In some aspects of the invention, atleast a portion of the anode exhaust from the MCFCs can be (afterseparation of CO₂) recycled to the input flow for the MCFC anode(s).Additionally or alternately, a portion of the anode exhaust from theMCFCs can be recycled to the input flow for the combustion reaction forpower generation. In one embodiment, a first portion of the anodeexhaust from the MCFCs (after separation of CO₂) can be recycled to theinput flow for the MCFC anode(s), and a second portion of the anodeexhaust from the MCFCs can be recycled to the input flow for thecombustion reaction for power generation. In aspects where the MCFCs canbe operated with remaining (unreacted) H₂ in the anode exhaust,recycling a portion of the H₂ from the anode exhaust to the anode inputcan reduce the fuel needed for operating the MCFCs. The portion of H₂delivered to the combustion reaction can advantageously modify and/orimprove reaction conditions for the combustion reaction, leading to moreefficient power generation. A water-gas shift reaction zone after theanode exhaust can optionally be used to further increase the amount ofH₂ present in the anode exhaust while also allowing conversion of COinto more easily separable CO₂.

In various aspects of the invention, an improved method for capturingCO₂ from a combustion source using a molten carbonate fuel cell can beprovided. This can include, for example, systems and methods for powergeneration using turbines (or other power or heat generation methodsbased on combustion, such as boilers, combustors, and/or catalyticoxidizers) while reducing and/or mitigating emissions during powergeneration. This can optionally be achieved, at least in part, by usinga combined cycle power generation system, where the flue gas and/or heatfrom combustion reaction(s) can also be used to power a steam turbine.This can additionally or alternately be achieved, at least in part, byusing one or more molten carbonate fuel cells (MCFCs) as both a carboncapture device as well as an additional source of electrical power. Insome aspects of the invention, the MCFCs can be operated under low fuelutilization conditions that can allow for improved carbon capture in thefuel cell while also reducing and/or minimizing the amount of fuel lostor wasted. Additionally or alternately, the MCFCs can be operated toreduce and/or minimize the total number and/or volume of MCFCs requiredto reduce the CO₂ content of a combustion flue gas stream to a desiredlevel, for example, 1.5 vol % or less or 1.0 vol % or less. In suchaspects, for the cathode output from the final cathode(s) in an arraysequence (typically at least including a series arrangement, or else thefinal cathode(s) and the initial cathode(s) would be the same), theoutput composition can include about 2.0 vol % or less of CO₂ (e.g.,about 1.5 vol % or less or about 1.2 vol % or less) and/or at leastabout 1.0 vol % of CO₂, such as at least about 1.2 vol % or at leastabout 1.5 vol %. Such aspects can be enabled, at least in part, byrecycling the exhaust from the anode back to the inlet of the anode,with removal of at least a portion of the CO₂ in the anode exhaust priorto returning the anode exhaust to the anode inlet. Such removal of CO₂from the anode exhaust can be achieved, for example, using a cryogenicCO₂ separator. In some optional aspects of the invention, the recycle ofanode exhaust to the anode inlet can be performed so that no pathway isprovided for the anode exhaust to be recycled directly to the cathodeinlet. By avoiding recycle of anode exhaust directly to the cathodeinlet, any CO₂ transported to the anode recycle loop via the MCFCs canremain in the anode recycle loop until the CO₂ is separated out from theother gases in the loop.

Molten carbonate fuel cells are conventionally used in a standalone modeto generate electricity. In a standalone mode, an input stream of fuel,such as methane, can be passed into the anode side of a molten carbonatefuel cell. The methane can be reformed (either externally or internally)to form H₂ and other gases. The H₂ can then be reacted with carbonateions that have crossed the electrolyte from the cathode in the fuel cellto form CO₂ and H₂O. For the reactions in the anode of the fuel cell,the rate of fuel utilization is typically about 70% or 75%, or evenhigher. In a conventional configuration, the remaining fuel in the anodeexhaust can be oxidized (burned) to generate heat for maintaining thetemperature of the fuel cell and/or external reformer, in view of theendothermic nature of the reforming reaction. Air and/or another oxygensource can be added during this oxidation to allow for more completecombustion. The anode exhaust (after oxidation) can then be passed intothe cathode. In this manner, a single fuel stream entering the anode canbe used to provide all of the energy and nearly all of the reactants forboth anode and cathode. This configuration can also allow all of thefuel entering the anode to be consumed while only requiring ˜70% or ˜75%or slightly more fuel utilization in the anode.

In the above standalone method, which can be typical of conventionalsystems, the goal of operating a molten carbonate fuel cell can begenerally to efficiently generate electric power based on an input fuelstream. By contrast, a molten carbonate fuel cell integrated with acombustion powered turbine, engine, or other generator can be used toprovide additional utility. Although high-efficiency power generation bythe fuel cell is still desirable, the fuel cell can be operated, forinstance, to improve and/or maximize the amount of CO₂ captured from anexhaust stream for a given volume of fuel cells. This can allow forimproved CO₂ capture while still generating power from the fuel cell.Additionally, in some aspects of the invention, the exhaust from theanode(s) of the fuel cell(s) can still contain excess hydrogen. Thisexcess hydrogen can advantageously be used as a fuel for the combustionreaction for the turbine, thus allowing for improved efficiency for theturbine.

FIG. 1 provides a schematic overview for the concept of some aspects ofthe invention. FIG. 1 is provided to aid in understanding of the generalconcept, so additional feeds, processes, and or configurations can beincorporated into FIG. 1 without departing from the spirit of theoverall concept. In the overview example shown in FIG. 1, a natural gasturbine 110 (or another combustion-powered turbine) can be used togenerate electric power based on combustion of a fuel 112. For thenatural gas turbine 110 shown in FIG. 1, this can include compressing anair stream or other gas phase stream 111 to form a compressed gas stream113. The compressed gas stream 113 can then be introduced into acombustion zone 115 along with fuel 112. Additionally, a stream 185,including a portion of the fuel (hydrogen) present in the exhaust fromanode 130, can also be introduced into the combustion zone 115. Thisadditional hydrogen can allow the combustion reaction to be operatedunder enhanced conditions. The resulting hot flue or exhaust gas 117 canthen be passed into the expander portion of turbine 110 to generateelectrical power.

After expansion (and optional clean up and/or other processing steps),the expanded flue gas can be passed into the cathode portion 120 of amolten carbonate fuel cell. The flue gas can include sufficient oxygenfor the reaction at the cathode, or additional oxygen can be provided ifnecessary. To facilitate the fuel cell reaction, fuel 132 can be passedinto the anode portion 130 of the fuel cell, along with at least aportion of the anode exhaust 135. Prior to being recycled, the anodeexhaust 135 can be passed through several additional processes. Oneadditional process can include or be a water-gas shift reaction process170. The water gas-shift reaction 170 can be used to react H₂O and COpresent in the anode exhaust 135 to form additional H₂ and CO₂. This canallow for improved removal of carbon from the anode exhaust 135, as CO₂can typically be more readily separated from the anode exhaust, ascompared to CO. The output 175 from the (optional) water-gas shiftprocess 170 can then be passed through a carbon dioxide separationsystem 140, such as a cryogenic carbon dioxide separator. This canremove at least a portion of CO₂ 147 from the anode exhaust, typicallyas well as a portion of the water 149 also. After removal of at least aportion of the CO₂ and water, the recycled anode exhaust can stillcontain some CO₂ and water, as well as unreacted fuel in the form of H₂and/or possibly a hydrocarbon such as methane. In certain embodiments ofthe invention, a portion 145 of the output from the CO₂ separationstage(s) can be recycled for use as an input stream to anode 130, whilea second portion can be used as input 185 to the combustion reaction115. Fuel 132 can represent a hydrogen-containing stream and/or a streamcontaining methane and/or another hydrocarbon that can be reformed(internally or externally) to form H₂.

The exhaust from the cathode portion 120 of the fuel cell can then bepassed into a heat recovery zone 150 so that heat from the cathodeexhaust can be recovered, e.g., to power a steam generator 160. Afterrecovering heat, the cathode exhaust can exit the system as an exhauststream 156. The exhaust stream 156 can exit to the environment, oroptional additional clean-up processes can be used, such as performingadditional CO₂ capture on stream 156, for example, using an aminescrubber.

One way of characterizing the operation of a fuel cell can be tocharacterize the “utilization” of various inputs received by the fuelcell. For example, one common method for characterizing the operation ofa fuel cell can be to specify the (anode) fuel utilization for the fuelcell.

In addition to fuel utilization, the utilization for other reactants inthe fuel cell can be characterized. For example, the operation of a fuelcell can additionally or alternately be characterized with regard to“CO₂ utilization” and/or “oxidant” utilization. The values for CO₂utilization and/or oxidant utilization can be specified in a similarmanner. For CO₂ utilization, the simplified calculation of (CO₂-rate-inminus CO₂-rate-out)/CO₂-rate-in can be used if CO₂ is the only fuelcomponent present in the input stream or flow to the cathode, with theonly reaction thus being the formation of CO₃ ²⁻. Similarly, for oxidantutilization, the simplified version can be used if O₂ is the onlyoxidant present in the input stream or flow to the cathode, with theonly reaction thus being the formation of CO₃ ²⁻.

Another reason for using a plurality of fuel cells can be to allow forefficient fuel cell operation while reducing the CO₂ content of thecombustion exhaust to a desired level. Rather than operating a fuel cellto have a high (or optimal) rate of CO₂ utilization, two (or more) fuelcells can be operated at lower fuel utilization rate(s) while reducingthe combustion to a desired level.

During conventional operation of a fuel cell, such as standaloneoperation, the goal of operating the fuel cell can be to generateelectrical power while efficiently using the “fuel” provided to thecell. The “fuel” can correspond to either hydrogen (H₂), a gas streamcomprising hydrogen, and/or a gas stream comprising a substance that canbe reformed to provide hydrogen (such as methane, another alkane orhydrocarbon, and/or one or more other types of compounds containingcarbon and hydrogen that, upon reaction, can provide hydrogen). Thesereforming reactions are typically endothermic and thus usually consumesome heat energy in the production of hydrogen. Carbon sources that canprovide CO directly and/or upon reaction can also be utilized, astypically the water gas shift reaction (CO+H₂O=H₂+CO₂) can occur in thepresence of the fuel cell anode catalyst surface. This can allow forproduction of hydrogen from a CO source. For such conventionaloperation, one potential goal of operating the fuel cell can be toconsume all of the fuel provided to the cell, while maintaining adesirable output voltage for the fuel cell, which can be traditionallyaccomplished by operating the fuel cell anode at a fuel utilization ofabout 70% to about 75%, followed by combusting (such as burning) theremaining fuel to generate heat to maintain the temperature of the fuelcell. The fuel utilization can be measured in terms of the totalenthalpy of the fuel used in the fuel cell reactions divided by theenthalpy of the fuel entering the fuel cell.

In a molten carbonate fuel cell, the transport of carbonate ions acrossthe electrolyte in the fuel cell can provide a method for transportingCO₂ from a first flow path to a second flow path, where the transportmethod can allow transport from a lower concentration (the cathode) to ahigher concentration (the anode), which can thus facilitate capture ofCO₂. Part of the selectivity of the fuel cell for CO₂ separation can bebased on the electrochemical reactions allowing the cell to generateelectrical power. For nonreactive species (such as N₂) that effectivelydo not participate in the electrochemical reactions within the fuelcell, there can be an insignificant amount of reaction and transportfrom cathode to anode. By contrast, the potential (voltage) differencebetween the cathode and anode can provide a strong driving force fortransport of carbonate ions across the fuel cell. As a result, thetransport of carbonate ions in the molten carbonate fuel cell can allowCO₂ to be transported from the cathode (lower CO₂ concentration) to theanode (higher CO₂ concentration) with relatively high selectivity.However, a challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. The voltageand/or power generated by a carbonate fuel cell can start to droprapidly as the CO₂ concentration falls below about 2.0 vol %. As the CO₂concentration drops further, e.g., to below about 1.0 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function. Thus, at least some CO₂ is likely to be present in theexhaust gas from the cathode stage of a fuel cell under commerciallyviable operating conditions.

The amount of carbon dioxide delivered to the fuel cell cathode(s) canbe determined based on the CO₂ content of a source for the cathodeinlet. One example of a suitable CO₂-containing stream for use as acathode input flow can be an output or exhaust flow from a combustionsource. Examples of combustion sources include, but are not limited to,sources based on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air). To a first approximation, the CO₂ content of the outputflow from a combustion source can be a minor portion of the flow. Evenfor a higher CO₂ content exhaust flow, such as the output from acoal-fired combustion source, the CO₂ content from most commercialcoal-fired power plants can be about 15 vol % or less. More generally,the CO₂ content of an output or exhaust flow from a combustion sourcecan be at least about 1.5 vol %, or at least about 1.6 vol %, or atleast about 1.7 vol %, or at least about 1.8 vol %, or at least about1.9 vol %, or at least greater 2 vol %, or at least about 4 vol %, or atleast about 5 vol %, or at least about 6 vol %, or at least about 8 vol%. Additionally or alternately, the CO₂ content of an output or exhaustflow from a combustion source can be about 20 vol % or less, such asabout 15 vol % or less, or about 12 vol % or less, or about 10 vol % orless, or about 9 vol % or less, or about 8 vol % or less, or about 7 vol% or less, or about 6.5 vol % or less, or about 6 vol % or less, orabout 5.5 vol % or less, or about 5 vol % or less, or about 4.5 vol % orless. The concentrations given above are on a dry basis. It is notedthat the lower CO₂ content values can be present in the exhaust fromsome natural gas or methane combustion sources, such as generators thatare part of a power generation system that may or may not include anexhaust gas recycle loop.

Operation of Anode Portion and Anode Recycle Loop

In various aspects of the invention, molten carbonate fuel cells can beoperated under conditions that allow for lower fuel utilization in theanode portion of the fuel cell. This can be in contrast to conventionaloperation for fuel cells, where the fuel utilization can be typicallyselected in order to allow a 70% or more of the fuel delivered to thefuel cell to be consumed as part of operation of the fuel cell. Inconventional operation, almost all of the fuel can be typically eitherconsumed within the anode of the fuel cell or burned to provide sensibleheat for the feed streams to the fuel cell.

FIG. 3 shows an example of the relationship between fuel utilization andoutput power for a fuel cell operating under conventional (stand-alone)conditions. The diagram shown in FIG. 3 shows two limiting cases foroperation of a fuel cell. One limiting case includes the limit ofoperating a fuel cell to consume an amount of fuel (such as H₂ ormethane reformed into H₂) that approaches 100% of the fuel delivered tothe fuel cell. From an efficiency standpoint, consumption of ˜100% ofthe fuel delivered to a fuel cell would be desirable, so as not to wastefuel during operation of the fuel cell. However, there are two potentialdrawbacks with operating a fuel cell to consume more than about 80% ofthe fuel delivered to the cell. First, as the amount of fuel consumedapproaches 100%, the voltage provided by the fuel cell can be sharplyreduced. In order to consume an amount of fuel approaching 100%, theconcentration of the fuel in the fuel cell (or at least near the anode)must almost by definition approach zero during at least part of theoperation of the fuel cell. Operating the anode of the fuel cell with afuel concentration approaching zero can result in a decreasingly lowdriving force for transporting carbonate across the electrolyte of thefuel cell. This can cause a corresponding drop in voltage, with thevoltage potentially also approaching zero in the true limiting case ofconsuming all fuel provided to the anode.

The second drawback is also related to relatively high fuel utilizationvalues (greater than about 80%). As shown in FIG. 3, at fuel utilizationvalues of about 75% or less, the voltage generated by the fuel cell hasa roughly linear relationship with the fuel utilization. In FIG. 3, atabout 75% fuel utilization, the voltage generated can be about 0.7Volts. In FIG. 3, at fuel utilization values of about 80% or greater,the voltage versus utilization curve appears to take on an exponentialor power type relationship. From a process stability standpoint, it canbe preferable to operate a fuel cell in a portion of the voltage versusutilization curve where the relationship is linear.

In the other limiting case shown in FIG. 3, the voltage generated by amolten carbonate fuel cell shows a mild increase as the fuel utilizationdecreases. However, in conventional operation, operating a fuel cell atreduced utilization can pose various difficulties. For example, thetotal amount of fuel delivered to a conventionally operated fuel celloperated with lower fuel utilization may need to be reduced, so thatwhatever fuel remains in the anode exhaust/output stream can stillprovide the appropriate amount of heat (upon further combustion) formaintaining the fuel cell temperature. If the fuel utilization isreduced without adjusting the amount of fuel delivered to the fuel cell,the oxidation of the unused fuel may result in higher than desiredtemperatures for the fuel cell. Based at least on these limiting caseconsiderations, conventional fuel cells are typically operated at a fuelutilization of about 70% to about 75%, so as to achieve heat balancewith complete utilization of the fuel.

An alternative configuration can be to recycle at least a portion of theexhaust from a fuel cell anode to the input of a fuel cell anode. Theoutput stream from an MCFC anode can include H₂O, CO₂, optionally CO,and optionally but typically unreacted fuel (such as H₂ or CH₄) as theprimary output components. Instead of using this output stream as a fuelsource to provide heat for a reforming reaction, one or more separationscan be performed on the anode output stream in order to separate out theCO₂ from the components with potential fuel value, such as H₂ and/or CO.The components with fuel value can then be recycled to the input of ananode.

This type of configuration can provide one or more benefits. First, CO₂can be separated out from the anode output, such as by using a cryogenicCO₂ separator. Several of the components of the anode output (H₂, CO,CH₄) are not easily condensable components, while CO₂ and H₂O can beseparated individually as condensed phases. Depending on the embodiment,at least about 90 vol % of the CO₂ in the anode output can be separatedout to form a relatively high purity CO₂ output stream. Afterseparation, the remaining portion of the anode output can correspondprimarily to components with fuel value, as well as reduced amounts ofCO₂ and/or H₂O. This portion of the anode output after separation can berecycled for use as part of the anode input, along with additional fuel.In this type of configuration, even though the fuel utilization in asingle pass through the MCFC(s) may be low, the unused fuel can beadvantageously recycled for another pass through the anode. As a result,the single-pass fuel utilization can be at a reduced level, whileavoiding loss (exhaust) of unburned fuel to the environment.

Additionally or alternatively to recycling a portion of the anodeexhaust to the anode input, another configuration option can be to use aportion of the anode exhaust as an input for a combustion reaction for aturbine or other combustion power source. The relative amounts of anodeexhaust recycled to the anode input and/or as an input to the combustionzone can be any convenient or desirable amount. If the anode exhaust isrecycled to only one of the anode input and the combustion zone, theamount of recycle can be any convenient amount, such as up to 100% ofthe portion of the anode exhaust remaining after any separation toremove CO₂ and/or H₂O. When a portion of the anode exhaust is recycledto both the anode input and the combustion zone, the total recycledamount by definition can be 100% or less of the remaining portion ofanode exhaust. Otherwise, any convenient split of the anode exhaust canbe used. In various embodiments of the invention, the amount of recycleto the anode input can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the anode input can be about 90% or less of theanode exhaust remaining after separations, for example about 75% orless, about 60% or less, about 50% or less, about 40% or less, about 25%or less, or about 10% or less. Further additionally or alternately, invarious embodiments of the invention, the amount of recycle to thecombustion zone (turbine) can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the combustion zone (turbine) can be about 90%or less of the anode exhaust remaining after separations, for exampleabout 75% or less, about 60% or less, about 50% or less, about 40% orless, about 25% or less, or about 10% or less.

Any H₂ present in the anode exhaust can represent a fuel that can becombusted without generating CO₂. Because at least some H₂ can begenerated as part of the anode portion of the fuel cell(s), the CO₂generated during reforming can be primarily removed by the CO₂separation stage(s) in the anode portion of the system. As a result, useof H₂ from the anode exhaust as part of the fuel for the combustionreaction can allow for a situation where the CO₂ generated from“combustion” of the fuel can be created in the anode portion of thesystem, as opposed to having to transport the CO₂ across the membrane.

Recycle of H₂ from the anode exhaust to the combustion reaction canprovide synergistic benefits for a turbine (or other combustion system)that include an exhaust gas recycle (EGR) configuration. In an EGRconfiguration, a portion of the CO₂-containing exhaust gas from thecombustion reaction can be recycled and used as part of the input gasflow to the turbine. During operation of a combustion-powered turbine,an input gas flow of an oxidant (such as air or oxygen-enriched air) canbe compressed prior to introduction into the combustion reaction. Thecompressors used for the input flows to the combustion reaction can tendto be volume limited, so that a similar number of moles of gas can becompressed, typically regardless of the mass of the gas. However, gaseswith a higher mass can tend to have higher heat capacities and/or canallow for greater pressure ratios across the expander portion of aturbine. A CO₂-enriched exhaust stream can provide a convenient sourceof a gas stream with higher molecular weight components that can allowfor improved conversion of the energy from the combustion reaction intoelectric power from the turbine. Although introducing a CO₂-enrichedstream into the combustion reaction can provide some benefits, there canbe effective limits to the amount of the CO₂-enriched stream that can beadded without significantly (negatively) impacting the combustionreaction. Since the CO₂-enriched stream does not itself typicallycontain “fuel”, the stream can largely act as a diluent within thecombustion reaction. As a result, the amount of recyclable CO₂ can belimited based, at least in part, on maintaining the conditions in thecombustion reaction within an appropriate flammability window.

Recycle of H₂ from the anode exhaust can complement an EGR configurationin one or more ways. First, combustion of H₂ may not directly result ingeneration of CO₂. Instead, as noted above, the CO₂ generated when theH₂ is produced can be generated in the anode loop. This can reduce theamount of CO₂ needing to be transferred from cathode to anode for agiven level of power generation. Additionally, H₂ can also have thebenefit of modifying the operation of the combustion source, such asthrough modifying the flammability window, so that higher concentrationsof CO₂ can be tolerated while still maintaining a desired combustionreaction. Being able to expand the flammability window can allow forincreased concentrations of CO₂ in the combustion exhaust, and thereforeincreased CO₂ in the input to the cathodes of the fuel cell.

The benefit of being able to increase the CO₂ concentration in the inputto the fuel cell cathode can be related to the nature of how a moltencarbonate fuel cell operates. As detailed below, there can be practicallimits in the amount of CO₂ separable by an MCFC from a cathode exhauststream. Depending on the operating conditions, an MCFC can lower the CO₂content of a cathode exhaust stream to about 2.0 vol % or less, e.g.,about 1.5 vol % or less or about 1.2 vol % or less. Due to thislimitation, the net efficiency of CO₂ removal when using moltencarbonate fuel cells can be dependent on the amount of CO₂ in thecathode input. For a combustion reaction using natural gas as a fuel,the amount of CO₂ in the combustion exhaust can correspond to a CO₂concentration at the cathode input of at least about 4 vol %. Use ofexhaust gas recycle can allow the amount of CO₂ at the cathode input tobe increased to at least about 5 vol %, e.g., at least about 6 vol %.Due to the increased flammability window that can be provided when usingH₂ as part of the fuel, the amount of CO₂ added via exhaust gas recyclecan be increased still further, so that concentrations of CO₂ at thecathode input of at least about 7.5 vol % or at least about 8 vol % canbe achieved. Based on a removal limit of about 1.5 vol % at the cathodeexhaust, increasing the CO₂ content at the cathode input from about 5.5vol % to about 7.5 vol % corresponds to a ˜50% increase in the amount ofCO₂ that can be captured using a fuel cell and transported to the anodeloop for eventual CO₂ separation.

The amount of H₂ present in the anode output can be increased, forexample, by using a water gas shift reactor to convert H₂O and COpresent in the anode output into H₂ and CO₂. Water is an expected outputof the reaction occurring at the anode, so the anode output cantypically have an excess of H₂O relative to the amount of CO present inthe anode output. CO can be present in the anode output due toincomplete carbon combustion during reforming and/or due to theequilibrium balancing reactions between H₂O, CO, H₂, and CO₂ (i.e., thewater-gas shift equilibrium) under either reforming conditions or theconditions present during the anode reaction. A water gas shift reactorcan be operated under conditions to drive the equilibrium further in thedirection of forming CO₂ and H₂ at the expense of CO and H₂O. Highertemperatures can tend to favor the formation of CO and H₂O. Thus, oneoption for operating the water gas shift reactor can be to expose theanode output stream to a suitable catalyst, such as a catalyst includingiron oxide, zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can also increase the amount of CO₂ at theexpense of CO. This can exchange difficult-to-remove carbon monoxide(CO) for carbon dioxide, which can be more readily removed bycondensation (e.g., cryogenic removal), chemical reaction (such as amineremoval), and/or other CO₂ removal methods.

In some aspects of the invention, all or substantially all of the anodeoutput stream remaining after separation of (portions of) the CO₂ (andH₂O) can be recycled for use as an input for the fuel cell anode(s)and/or as a fuel input for the combustion-powered generator. Thus, forthe portion of the anode output stream that remains after a water-gasshift reaction, removal of CO₂, and/or removal of H₂O, at least about90% of the remaining content can advantageously be used as either aninput for the fuel cell anode(s) or as a fuel input for the combustionpowered generator. Alternatively, the anode output stream afterseparation can be used for more than one purpose, but recycle of anyportion of the anode output stream for use as a direct input to acathode and/or as an input to an oxidizer for heating of the fuel cellcan advantageously be avoided.

FIG. 4 shows an example of the anode flow path portion of agenerator/fuel cell system according to the invention. In FIG. 4, aninitial fuel stream 405 can optionally be reformed 410 to convertmethane (or another type of fuel) and water into H₂ and CO₂.Alternatively, the reforming reaction can be performed in a reformingstage that is part of an assembly including both the reforming stage andthe fuel cell anode 420. Additionally or alternately, at least a portionof fuel stream 405 can correspond to hydrogen gas, so that the amount ofreforming needed to provide fuel to the anode 420 can be reduced and/orminimized. The optionally reformed fuel 415 can then be passed intoanode 420. A recycle stream 455 including fuel components from the anodeexhaust 425 can also serve as an input to the anode 420. A flow ofcarbonate ions 422 from the cathode portion of the fuel cell (not shown)can provide the remaining reactant needed for the anode fuel cellreactions. Based on the reactions in the anode 420, the resulting anodeexhaust 425 can include H₂O, CO₂, one or more components correspondingto unreacted fuel (H₂, CO, CH₄, or others), and optionally one or moreadditional non-reactive components, such as N₂ and/or other contaminantsthat are part of fuel stream 405. The anode exhaust 425 can then bepassed into one or more separation stages 430 for removal of CO₂ (andoptionally also H₂O). A cryogenic CO₂ removal system can be an exampleof a suitable type of separation stage. Optionally, the anode exhaustcan first be passed through a water gas shift reactor 440 to convert anyCO present in the anode exhaust (along with some H₂O) into CO₂ and H₂ inan optionally water gas shifted anode exhaust 445.

An initial portion of the separation stage(s) 430 can be used to removea majority of the H₂O present in the anode exhaust 425 as an H₂O outputstream 432. Additionally or alternately, a heat recovery steam generatorsystem or other heat exchangers independent of the cryogenic separationsystem can be used to remove a portion of the H₂O. A cryogenic CO₂removal system can then remove a majority of the CO₂ as a high purityCO₂ stream 434. A purge stream (not shown) can also be present, ifdesired, to prevent accumulation of inert gases within the anode recycleloop. The remaining components of the anode exhaust stream can then beused either as a recycled input 455 to the inlet of anode 420 or as aninput stream 485 for a combustion powered turbine.

Conventionally, at least some reforming is performed prior to any fuelentering a fuel cell. This initial/preliminary reforming can beperformed in a reformer that is external to the fuel cell(s) or fuelcell stack(s). Alternatively, the assembly for a fuel cell stack caninclude one or more reforming zones located within the stack but priorto the anodes of the fuel cells in the stack. This initial reformingtypically converts at least some fuel into hydrogen prior to enteringthe anode, so that the stream that enters the anode can have sufficienthydrogen to maintain the anode reaction. Without this initial reforming,in certain embodiments, the hydrogen content in the anode can be toolow, resulting in little or no transport of CO₂ from cathode to anode.By contrast, in some embodiments the fuel cell(s) in a fuel cell arraycan be operated without external reforming, i.e., based only onreforming within the anode portion of the fuel cell, due to sufficienthydrogen being present in the recycled portion of the anode exhaust.When a sufficient amount of H₂ is present in the anode feed, such as atleast about 10 vol % of the fuel delivered to the anode in the form ofH₂, the reaction conditions in the anode can allow for additionalreforming to take place within the anode itself, which, depending onflow path, can reduce and/or eliminate the need for a reforming stageexternal (prior) to the anode input(s) in the methods according to theinvention. If the anode feed does not contain a sufficient amount ofhydrogen, the anode reaction can stall, and reforming activity and/orother reactions in the anode can be reduced, minimized, or haltedentirely. As a result, in embodiments where the amount of H₂ present inthe anode feed is insufficient, it may be desirable (or necessary) forthere to be a reforming stage external (prior) to the anode input(s).

Operation of Cathode Portion

In various aspects according to the invention, molten carbonate fuelcells used for carbon capture can be operated to improve or enhance thecarbon capture aspects of the fuel cells, as opposed to (or even at theexpense of) enhancing the power generation capabilities. Conventionally,a molten carbonate fuel cell can be operated based on providing adesirable voltage while consuming all fuel in the fuel stream deliveredto the anode. This can be conventionally achieved in part by using theanode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention uses separate/different sources for theanode input and cathode input. By removing the link between thecomposition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell toimprove capture of carbon dioxide.

One initial challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. FIG. 5 showsan example of the relationship between 1) voltage and CO₂ concentrationand 2) power and CO₂ concentration, based on the concentration of CO₂ inthe cathode input gas. As shown in FIG. 5, the voltage and/or powergenerated by a carbonate fuel cell can start to drop rapidly as the CO₂concentration falls below about 2.0 vol %. As the CO₂ concentrationdrops further, e.g., to below 1.0 vol %, at some point the voltageacross the fuel cell can become low enough that little or no furthertransport of carbonate may occur. Thus, at least some CO₂ is likely tobe present in the exhaust gas from the cathode stage of a fuel cell,pretty much regardless of the operating conditions.

One modification of the fuel cell operating conditions can be to operatethe fuel cell with an excess of available reactants at the anode, suchas by operating with low fuel utilization at the anode, as describedabove. By providing an excess of the reactants for the anode reaction inthe fuel cell, the availability of CO₂ for the cathode reaction can beused as a/the rate limiting variable for the reaction.

When operating MCFCs to enhance the amount of carbon capture, thefactors for balancing can be different than when attempting to improvefuel utilization. In particular, the amount of carbon dioxide deliveredto the fuel cells can be determined based on the output flow from thecombustion generator providing the CO₂-containing stream. To a firstapproximation, the CO₂ content of the output flow from a combustiongenerator can be a minor portion of the flow. Even for a higher CO₂content exhaust flow, such as the output from a coal-fired combustiongenerator, the CO₂ content from most commercial coal fired power plantscan be about 15 vol % or less. In order to perform the cathode reaction,this could potentially include between about 5% and about 15%, typicallybetween about 7% and about 9%, of oxygen used to react with the CO₂ toform carbonate ions. As a result, less than about 25 vol % of the inputstream to the cathode can typically be consumed by the cathodereactions. The remaining at least about 75% portion of the cathode flowcan be comprised of inert/non-reactive species such as N₂, H₂O, andother typical oxidant (air) components, along with any unreacted CO₂ andO₂.

Based on the nature of the input flow to the cathode relative to thecathode reactions, the portion of the cathode input consumed and removedat the cathode can be about 25 vol % or less, for example about 10 vol %or less for input flows based on combustion of cleaner fuel sources,such as natural gas sources. The exact amount can vary based on the fuelused, the diluent content in the input fuel (e.g., N₂ is typicallypresent in natural gas at a small percentage), and the oxidant (air) tofuel ratio at which the combustor is operated, all of which can vary,but are typically well known for commercial operations. As a result, thetotal gas flow into the cathode portions of the fuel cells can berelatively predictable (constant) across the total array of fuel cellsused for carbon capture. Several possible configurations can be used inorder to provide an array of fuel cells to enhance/improve/optimizecarbon capture. The following configuration options can be used alone orin combination as part of the strategy for improving carbon capture.

A first configuration option can be to divide the CO₂-containing streambetween a plurality of fuel cells. The CO₂-containing output stream froman industrial generator can typically correspond to a large flow volumerelative to desirable operating conditions for a single MCFC ofreasonable size. Instead of processing the entire flow in a single MCFC,the flow can be divided amongst a plurality of MCFC units, usually atleast some of which are in parallel, so that the flow rate in each unitcan be within a desired flow range.

A second configuration option can be to utilize fuel cells in series tosuccessively remove CO₂ from a flow stream. Regardless of the number ofinitial fuel cells to which a CO₂-containing stream can be distributedto in parallel, each initial fuel cell can be followed by one or moreadditional cells in series to further remove additional CO₂. Similar tothe situation demonstrated in FIG. 3 for the H₂ input to the anode,attempting to remove CO₂ within a stream in a single fuel cell couldlead to a low and/or unpredictable voltage output. Rather thanattempting to remove CO₂ to a desired level in a single fuel cell, CO₂can be removed in successive cells until a desired level can beachieved. For example, each cell in a series of fuel cells can be usedto remove some percentage (e.g., about 50%) of the CO₂ present in a fuelstream. In such an example, if three fuel cells are used in series, theCO₂ concentration can be reduced (e.g., to about 15% or less of theoriginal amount present, which can correspond to reducing the CO₂concentration from about 6% to about 1% or less over the course of threefuel cells in series).

In another configuration, the operating conditions can be selected inearly fuel stages in series to provide a desired output voltage whilethe array of stages can be selected to achieve a desired level of carboncapture. As an example, an array of fuel cells can be used with threefuel cells in series. The first two fuel cells in series can be used toremove CO₂ while maintaining a desired output voltage. The final fuelcell can then be operated to remove CO₂ to a desired concentration.

In still another configuration, there can be separate connectivity forthe anodes and cathodes in a fuel cell array. For example, if the fuelcell array includes fuel cathodes connected in series, the correspondinganodes can be connected in any convenient manner, not necessarilymatching up with the same arrangement as their corresponding cathodes,for example. This can include, for instance, connecting the anodes inparallel, so that each anode receives the same type of fuel feed, and/orconnecting the anodes in a reverse series, so that the highest fuelconcentration in the anodes can correspond to those cathodes having thelowest CO₂ concentration.

Hydrogen or Syngas Capture

Either hydrogen or syngas can be withdrawn from the anode exhaust as achemical energy output. Hydrogen can be used as a clean fuel withoutgenerating greenhouse gases when it is burned or combusted. Instead, forhydrogen generated by reforming of hydrocarbons (or hydrocarbonaceouscompounds), the CO₂ will have already been “captured” in the anode loop.Additionally, hydrogen can be a valuable input for a variety of refineryprocesses and/or other synthesis processes. Syngas can also be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a feedstock for producing other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes.

In various aspects, the anode exhaust can have a ratio of H₂ to CO ofabout 1.5:1 to about 10:1, such as at least about 3.0:1, or at leastabout 4.0:1, or at least about 5.0:1, and/or about 8.0:1 or less orabout 6.0:1 or less. A syngas stream can be withdrawn from the anodeexhaust. In various aspects, a syngas stream withdrawn from an anodeexhaust can have a ratio of moles of H₂ to moles of CO of at least about0.9:1, such as at least about 1.0:1, or at least about 1.2:1, or atleast about 1.5:1, or at least about 1.7:1, or at least about 1.8:1, orat least about 1.9:1. Additionally or alternately, the molar ratio of H₂to CO in a syngas withdrawn from an anode exhaust can be about 3.0:1 orless, such as about 2.7:1 or less, or about 2.5:1 or less, or about2.3:1 or less, or about 2.2:1 or less, or about 2.1:1 or less. It isnoted that higher ratios of H₂ to CO in a withdrawn syngas stream cantend to reduce the amount of CO relative to the amount of CO₂ in acathode exhaust. However, many types of syngas applications benefit fromsyngas with a molar ratio of H₂ to CO of at least about 1.5:1 to about2.5:1 or less, so forming a syngas stream with a molar ratio of H₂ to COcontent of, for example, about 1.7:1 to about 2.3:1 may be desirable forsome applications.

Syngas can be withdrawn from an anode exhaust by any convenient method.In some aspects, syngas can be withdrawn from the anode exhaust byperforming separations on the anode exhaust to remove at least a portionof the components in the anode exhaust that are different from H₂ andCO. For example, an anode exhaust can first be passed through anoptional water-gas shift stage to adjust the relative amounts of H₂ andCO. One or more separation stages can then be used to remove H₂O and/orCO₂ from the anode exhaust. The remaining portion of the anode exhaustcan then correspond to the syngas stream, which can then be withdrawnfor use in any convenient manner. Additionally or alternately, thewithdrawn syngas stream can be passed through one or more water-gasshift stages and/or passed through one or more separation stages.

It is noted that an additional or alternative way of modifying the molarratio of H₂ to CO in the withdrawn syngas can be to separate an H₂stream from the anode exhaust and/or the syngas, such as by performing amembrane separation. Such a separation to form a separate H₂ outputstream can be performed at any convenient location, such as prior toand/or after passing the anode exhaust through a water-gas shiftreaction stage, and prior to and/or after passing the anode exhaustthrough one or more separation stages for removing components in theanode exhaust different from H₂ and CO. Optionally, a water-gas shiftstage can be used both before and after separation of an H₂ stream fromthe anode exhaust. In an additional or alternative embodiment, H₂ canoptionally be separated from the withdrawn syngas stream. In someaspects, a separated H₂ stream can correspond to a high purity H₂stream, such as an H₂ stream containing at least about 90 vol % of H₂,such as at least about 95 vol % of H₂ or at least about 99 vol % of H₂.

In some aspects, a molten carbonate fuel cell can be operated using acathode input feed with a moderate or low CO₂ content. A variety ofstreams that are desirable for carbon separation and capture can includestreams with moderate to low CO₂ content. For example, a potential inputstream for a cathode inlet can have a CO₂ content of about 20 vol % orless, such as about 15 vol % or less, or about 12 vol % or less, orabout 10 vol % or less. Such a CO₂-containing stream can be generated bya combustion generator, such as a coal-fired or natural gas-firedturbine. Achieving a desired level of CO₂ utilization on a cathode inputstream with a moderate or low CO₂ content can allow for use of a lowercontent CO₂ stream, as opposed to needing to enrich a stream with CO₂prior to using the stream as a cathode input stream. In various aspects,the CO₂ utilization for a fuel cell can be at least about 50%, such asat least about 55% or at least about 60%. Additionally or alternately,the CO₂ utilization can be about 98% or less, such as about 97% or less,or about 95% or less, or about 90% or less, or alternatively can be justhigh enough so that sufficient CO₂ remains in the cathode exhaust toallow efficient or desired operation of the fuel cell. As used herein,CO₂ utilization may be the difference between the moles of CO₂ in thecathode outlet stream and the moles of CO₂ in the cathode inlet streamdivided by the moles of CO₂ in the cathode inlet. Expressedmathematically, CO₂utilization=(CO_(2(cathode in))−CO_(2(cathode out)))/CO_(2(cathode in)).

Operating Strategies

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated with increased productionof syngas (or hydrogen) while also reducing or minimizing the amount ofCO₂ exiting the fuel cell in the cathode exhaust stream. Syngas can be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a raw material for forming other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes. One option for makingsyngas can be to reform a hydrocarbon or hydrocarbon-like fuel, such asmethane or natural gas. For many types of industrial processes, a syngashaving a ratio of H₂ to CO of close to 2:1 (or even lower) can often bedesirable. A water gas shift reaction can be used to reduce the H₂ to COratio in a syngas if additional CO₂ is available, such as is produced inthe anodes.

One way of characterizing the overall benefit provided by integratingsyngas generation with use of molten carbonate fuel cells can be basedon a ratio of the net amount of syngas that exits the fuel cells in theanode exhaust relative to the amount of CO₂ that exits the fuel cells inthe cathode exhaust. This characterization measures the effectiveness ofproducing power with low emissions and high efficiency (both electricaland chemical). In this description, the net amount of syngas in an anodeexhaust is defined as the combined number of moles of H₂ and number ofmoles of CO present in the anode exhaust, offset by the amount of H₂ andCO present in the anode inlet. Because the ratio is based on the netamount of syngas in the anode exhaust, simply passing excess H₂ into theanode does not change the value of the ratio. However, H₂ and/or COgenerated due to reforming in the anode and/or in an internal reformingstage associated with the anode can lead to higher values of the ratio.Hydrogen oxidized in the anode can lower the ratio. It is noted that thewater gas shift reaction can exchange H₂ for CO, so the combined molesof H₂ and CO represents the total potential syngas in the anode exhaust,regardless of the eventual desired ratio of H₂ to CO in a syngas. Thesyngas content of the anode exhaust (H₂+CO) can then be compared withthe CO₂ content of the cathode exhaust. This can provide a type ofefficiency value that can also account for the amount of carbon capture.This can equivalently be expressed as an equation as

Ratio of net syngas in anode exhaust to cathode CO₂=net moles of(H₂+CO)_(ANODE)/moles of (CO₂)_(CATHODE)

In various aspects, the ratio of net moles of syngas in the anodeexhaust to the moles of CO₂ in the cathode exhaust can be at least about2.0, such as at least about 3.0, or at least about 4.0, or at leastabout 5.0. In some aspects, the ratio of net syngas in the anode exhaustto the amount of CO₂ in the cathode exhaust can be still higher, such asat least about 10.0, or at least about 15.0, or at least about 20.0.Ratio values of about 40.0 or less, such as about 30.0 or less, or about20.0 or less, can additionally or alternately be achieved. In aspectswhere the amount of CO₂ at the cathode inlet is about 6.0 volume % orless, such as about 5.0 volume % or less, ratio values of at least about1.5 may be sufficient/realistic. Such molar ratio values of net syngasin the anode exhaust to the amount of CO₂ in the cathode exhaust can begreater than the values for conventionally operated fuel cells.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at a reduced fuelutilization value, such as a fuel utilization of about 50% or less,while also having a high CO₂ utilization value, such as at least about60%. In this type of configuration, the molten carbonate fuel cell canbe effective for carbon capture, as the CO₂ utilization canadvantageously be sufficiently high. Rather than attempting to maximizeelectrical efficiency, in this type of configuration the totalefficiency of the fuel cell can be improved or increased based on thecombined electrical and chemical efficiency. The chemical efficiency canbe based on withdrawal of a hydrogen and/or syngas stream from the anodeexhaust as an output for use in other processes. Even though theelectrical efficiency may be reduced relative to some conventionalconfigurations, making use of the chemical energy output in the anodeexhaust can allow for a desirable total efficiency for the fuel cell.

In various aspects, the fuel utilization in the fuel cell anode can beabout 50% or less, such as about 40% or less, or about 30% or less, orabout 25% or less, or about 20% or less. In various aspects, in order togenerate at least some electric power, the fuel utilization in the fuelcell can be at least about 5%, such as at least about 10%, or at leastabout 15%, or at least about 20%, or at least about 25%, or at leastabout 30%. Additionally or alternatively, the CO₂ utilization can be atleast about 60%, such as at least about 65%, or at least about 70%, orat least about 75%.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell canbe operated so that the amount of reforming can be selected relative tothe amount of oxidation in order to achieve a desired thermal ratio forthe fuel cell. As used herein, the “thermal ratio” is defined as theheat produced by exothermic reactions in a fuel cell assembly divided bythe endothermic heat demand of reforming reactions occurring within thefuel cell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(EX)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions corresponds to any heat due toreforming reactions, water gas shift reactions, and the electrochemicalreactions in the cell. The heat generated by the electrochemicalreactions can be calculated based on the ideal electrochemical potentialof the fuel cell reaction across the electrolyte minus the actual outputvoltage of the fuel cell. For example, the ideal electrochemicalpotential of the reaction in a MCFC is believed to be about 1.04V basedon the net reaction that occurs in the cell. During operation of theMCFC, the cell will typically have an output voltage less than 1.04 Vdue to various losses. For example, a common output/operating voltagecan be about 0.7 V. The heat generated is equal to the electrochemicalpotential of the cell (i.e. ˜1.04V) minus the operating voltage. Forexample, the heat produced by the electrochemical reactions in the cellis ˜0.34 V when the output voltage of ˜0.7V. Thus, in this scenario, theelectrochemical reactions would produce ˜0.7 V of electricity and ˜0.34V of heat energy. In such an example, the ˜0.7 V of electrical energy isnot included as part of Q_(EX). In other words, heat energy is notelectrical energy.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a single anode within a single fuel cell, an anodesection within a fuel cell stack, or an anode section within a fuel cellstack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe anode section to be integrated from a heat integration standpoint.As used herein, “an anode section” comprises anodes within a fuel cellstack that share a common inlet or outlet manifold.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a molten carbonate fuel cell can beoperated to have a thermal ratio of about 1.5 or less, for example about1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50. Additionally or alternately, in someaspects the fuel cell can be operated to have a temperature rise betweenanode input and anode output of about 40° C. or less, such as about 20°C. or less, or about 10° C. or less. Further additionally oralternately, the fuel cell can be operated to have an anode outlettemperature that is from about 10° C. lower to about 10° C. higher thanthe temperature of the anode inlet. Still further additionally oralternately, the fuel cell can be operated to have an anode inlettemperature that is greater than the anode outlet temperature, such asat least about 5° C. greater, or at least about 10° C. greater, or atleast about 20° C. greater, or at least about 25° C. greater. Yet stillfurther additionally or alternately, the fuel cell can be operated tohave an anode inlet temperature that is greater than the anode outlettemperature by about 100° C. or less, such as by about 80° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less, or about 20° C. or less.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated with an excess ofreformable fuel relative to the amount of hydrogen reacted in the anodeof the fuel cell. Instead of selecting the operating conditions of afuel cell to improve or maximize the electrical efficiency of the fuelcell, an excess of reformable fuel can be passed into the anode of thefuel cell to increase the chemical energy output of the fuel cell.Optionally but preferably, this can lead to an increase in the totalefficiency of the fuel cell based on the combined electrical efficiencyand chemical efficiency of the fuel cell.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than theamount of hydrogen oxidized in the anode, such as at least about 75%greater or at least about 100% greater. In various aspects, a ratio ofthe reformable hydrogen content of the reformable fuel in the fuelstream relative to an amount of hydrogen reacted in the anode can be atleast about 1.5:1, or at least about 2.0:1, or at least about 2.5:1, orat least about 3.0:1. Additionally or alternately, the ratio ofreformable hydrogen content of the reformable fuel in the fuel streamrelative to the amount of hydrogen reacted in the anode can be about20:1 or less, such as about 15:1 or less or about 10:1 or less. In oneaspect, it is contemplated that less than 100% of the reformablehydrogen content in the anode inlet stream can be converted to hydrogen.For example, at least about 80% of the reformable hydrogen content in ananode inlet stream can be converted to hydrogen in the anode and/or inan associated reforming stage, such as at least about 85%, or at leastabout 90%.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can also be operated at conditions thatcan improve or optimize the combined electrical efficiency and chemicalefficiency of the fuel cell. Instead of selecting conventionalconditions for maximizing the electrical efficiency of a fuel cell, theoperating conditions can allow for output of excess synthesis gas and/orhydrogen in the anode exhaust of the fuel cell. The synthesis gas and/orhydrogen can then be used in a variety of applications, includingchemical synthesis processes and collection of hydrogen for use as a“clean” fuel. In aspects of the invention, electrical efficiency can bereduced to achieve a high overall efficiency, which includes a chemicalefficiency based on the chemical energy value of syngas and/or hydrogenproduced relative to the energy value of the fuel input for the fuelcell.

In some aspects, the operation of the fuel cells can be characterizedbased on electrical efficiency. Where fuel cells are operated to have alow electrical efficiency (EE), a molten carbonate fuel cell can beoperated to have an electrical efficiency of about 40% or less, forexample, about 35% EE or less, about 30% EE or less, about 25% EE orless, or about 20% EE or less, about 15% EE or less, or about 10% EE orless. Additionally or alternately, the EE can be at least about 5%, orat least about 10%, or at least about 15%, or at least about 20%.Further additionally or alternately, the operation of the fuel cells canbe characterized based on total fuel cell efficiency (TFCE), such as acombined electrical efficiency and chemical efficiency of the fuelcell(s). Where fuel cells are operated to have a high total fuel cellefficiency, a molten carbonate fuel cell can be operated to have a TFCE(and/or combined electrical efficiency and chemical efficiency) of about55% or more, for example, about 60% or more, or about 65% or more, orabout 70% or more, or about 75% or more, or about 80% or more, or about85% or more. It is noted that for a total fuel cell efficiency and/orcombined electrical efficiency and chemical efficiency, any additionalelectricity generated from use of excess heat generated by the fuel cellcan be excluded from the efficiency calculation.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired electrical efficiency of about 40%or less and a desired total fuel cell efficiency of about 55% or more.Where fuel cells are operated to have a desired electrical efficiencyand a desired total fuel cell efficiency, a molten carbonate fuel cellcan be operated to have an electrical efficiency of about 40% or lesswith a TFCE of about 55% or more, for example, about 35% EE or less withabout a TFCE of 60% or more, about 30% EE or less with about a TFCE ofabout 65% or more, about 25% EE or less with about a 70% TFCE or more,or about 20% EE or less with about a TFCE of 75% or more, about 15% EEor less with about a TFCE of 80% or more, or about 10% EE or less withabout a TFCE of about 85% or more.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at conditions that canprovide increased power density. The power density of a fuel cellcorresponds to the actual operating voltage V_(A) multiplied by thecurrent density I. For a molten carbonate fuel cell operating at avoltage V_(A), the fuel cell also can tend to generate waste heat, thewaste heat defined as (V_(O)−V_(A))*I based on the differential betweenV_(A) and the ideal voltage V_(O) for a fuel cell providing currentdensity I. A portion of this waste heat can be consumed by reforming ofa reformable fuel within the anode of the fuel cell. The remainingportion of this waste heat can be absorbed by the surrounding fuel cellstructures and gas flows, resulting in a temperature differential acrossthe fuel cell. Under conventional operating conditions, the powerdensity of a fuel cell can be limited based on the amount of waste heatthat the fuel cell can tolerate without compromising the integrity ofthe fuel cell.

In various aspects, the amount of waste heat that a fuel cell cantolerate can be increased by performing an effective amount of anendothermic reaction within the fuel cell. One example of an endothermicreaction includes steam reforming of a reformable fuel within a fuelcell anode and/or in an associated reforming stage, such as anintegrated reforming stage in a fuel cell stack. By providing additionalreformable fuel to the anode of the fuel cell (or to anintegrated/associated reforming stage), additional reforming can beperformed so that additional waste heat can be consumed. This can reducethe amount of temperature differential across the fuel cell, thusallowing the fuel cell to operate under an operating condition with anincreased amount of waste heat. The loss of electrical efficiency can beoffset by the creation of an additional product stream, such as syngasand/or H₂, that can be used for various purposes including additionalelectricity generation further expanding the power range of the system.

In various aspects, the amount of waste heat generated by a fuel cell,(V₀−V_(A))*I as defined above, can be at least about 30 mW/cm², such asat least about 40 mW/cm², or at least about 50 mW/cm², or at least about60 mW/cm², or at least about 70 mW/cm², or at least about 80 mW/cm², orat least about 100 mW/cm², or at least about 120 mW/cm², or at leastabout 140 mW/cm², or at least about 160 mW/cm², or at least about 180mW/cm². Additionally or alternately, the amount of waste heat generatedby a fuel cell can be less than about 250 mW/cm², such as less thanabout 200 mW/cm², or less than about 180 mW/cm², or less than about 165mW/cm², or less than about 150 mW/cm².

Although the amount of waste heat being generated can be relativelyhigh, such waste heat may not necessarily represent operating a fuelcell with poor efficiency. Instead, the waste heat can be generated dueto operating a fuel cell at an increased power density. Part ofimproving the power density of a fuel cell can include operating thefuel cell at a sufficiently high current density. In various aspects,the current density generated by the fuel cell can be at least about 150mA/cm², such as at least about 160 mA/cm², or at least about 170 mA/cm²,or at least about 180 mA/cm², or at least about 190 mA/cm², or at leastabout 200 mA/cm², or at least about 225 mA/cm², or at least about 250mA/cm². Additionally or alternately, the current density generated bythe fuel cell can be about 500 mA/cm² or less, such as 450 mA/cm², orless, or 400 mA/cm², or less or 350 mA/cm², or less or 300 mA/cm² orless.

In various aspects, to allow a fuel cell to be operated with increasedpower generation and increased generation of waste heat, an effectiveamount of an endothermic reaction (such as a reforming reaction) can beperformed. Alternatively, other endothermic reactions unrelated to anodeoperations can be used to utilize the waste heat by interspersing“plates” or stages into the fuel cell array in thermal communication butnot in fluid communication with either anodes or cathodes. The effectiveamount of the endothermic reaction can be performed in an associatedreforming stage, an integrated reforming stage, an integrated stackelement for performing an endothermic reaction, or a combinationthereof. The effective amount of the endothermic reaction can correspondto an amount sufficient to reduce the temperature rise from the fuelcell inlet to the fuel cell outlet to about 100° C. or less, such asabout 90° C. or less, or about 80° C. or less, or about 70° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less. Additionally or alternately, theeffective amount of the endothermic reaction can correspond to an amountsufficient to cause a temperature decrease from the fuel cell inlet tothe fuel cell outlet of about 100° C. or less, such as about 90° C. orless, or about 80° C. or less, or about 70° C. or less, or about 60° C.or less, or about 50° C. or less, or about 40° C. or less, or about 30°C. or less, or about 20° C. or less, or about 10° C. or less. Atemperature decrease from the fuel cell inlet to the fuel cell outletcan occur when the effective amount of the endothermic reaction exceedsthe waste heat generated. Additionally or alternately, this cancorrespond to having the endothermic reaction(s) (such as a combinationof reforming and another endothermic reaction) consume at least about40% of the waste heat generated by the fuel cell, such as consuming atleast about 50% of the waste heat, or at least about 60% of the wasteheat, or at least about 75% of the waste heat. Further additionally oralternately, the endothermic reaction(s) can consume about 95% of thewaste heat or less, such as about 90% of the waste heat or less, orabout 85% of the waste heat or less.

DEFINITIONS

Syngas: In this description, syngas is defined as mixture of H₂ and COin any ratio. Optionally, H₂O and/or CO₂ may be present in the syngas.Optionally, inert compounds (such as nitrogen) and residual reformablefuel compounds may be present in the syngas. If components other than H₂and CO are present in the syngas, the combined volume percentage of H₂and CO in the syngas can be at least 25 vol % relative to the totalvolume of the syngas, such as at least 40 vol %, or at least 50 vol %,or at least 60 vol %. Additionally or alternately, the combined volumepercentage of H₂ and CO in the syngas can be 100 vol % or less, such as95 vol % or less or 90 vol % or less.

Reformable fuel: A reformable fuel is defined as a fuel that containscarbon-hydrogen bonds that can be reformed to generate H₂. Hydrocarbonsare examples of reformable fuels, as are other hydrocarbonaceouscompounds such as alcohols. Although CO and H₂O can participate in awater gas shift reaction to form hydrogen, CO is not considered areformable fuel under this definition.

Reformable hydrogen content: The reformable hydrogen content of a fuelis defined as the number of H₂ molecules that can be derived from a fuelby reforming the fuel and then driving the water gas shift reaction tocompletion to maximize H₂ production. It is noted that H₂ by definitionhas a reformable hydrogen content of 1, although H₂ itself is notdefined as a reformable fuel herein. Similarly, CO has a reformablehydrogen content of 1. Although CO is not strictly reformable, drivingthe water gas shift reaction to completion will result in exchange of aCO for an H₂. As examples of reformable hydrogen content for reformablefuels, the reformable hydrogen content of methane is 4 H₂ moleculeswhile the reformable hydrogen content of ethane is 7 H₂ molecules. Moregenerally, if a fuel has the composition CxHyOz, then the reformablehydrogen content of the fuel at 100% reforming and water-gas shift isn(H₂ max reforming)=2x+y/2−z. Based on this definition, fuel utilizationwithin a cell can then be expressed as n(H₂ ox)/n(H₂ max reforming) Ofcourse, the reformable hydrogen content of a mixture of components canbe determined based on the reformable hydrogen content of the individualcomponents. The reformable hydrogen content of compounds that containother heteroatoms, such as oxygen, sulfur or nitrogen, can also becalculated in a similar manner.

Oxidation Reaction: In this discussion, the oxidation reaction withinthe anode of a fuel cell is defined as the reaction corresponding tooxidation of H₂ by reaction with CO₃ ²⁻ to form H₂O and CO₂. It is notedthat the reforming reaction within the anode, where a compoundcontaining a carbon-hydrogen bond is converted into H₂ and CO or CO₂, isexcluded from this definition of the oxidation reaction in the anode.The water-gas shift reaction is similarly outside of this definition ofthe oxidation reaction. It is further noted that references to acombustion reaction are defined as references to reactions where H₂ or acompound containing carbon-hydrogen bond(s) are reacted with O₂ to formH₂O and carbon oxides in a non-electrochemical burner, such as thecombustion zone of a combustion-powered generator.

Aspects of the invention can adjust anode fuel parameters to achieve adesired operating range for the fuel cell. Anode fuel parameters can becharacterized directly, and/or in relation to other fuel cell processesin the form of one or more ratios. For example, the anode fuelparameters can be controlled to achieve one or more ratios including afuel utilization, a fuel cell heating value utilization, a fuel surplusratio, a reformable fuel surplus ratio, a reformable hydrogen contentfuel ratio, and combinations thereof.

Fuel utilization: Fuel utilization is an option for characterizingoperation of the anode based on the amount of oxidized fuel relative tothe reformable hydrogen content of an input stream can be used to definea fuel utilization for a fuel cell. In this discussion, “fuelutilization” is defined as the ratio of the amount of hydrogen oxidizedin the anode for production of electricity (as described above) versusthe reformable hydrogen content of the anode input (including anyassociated reforming stages). Reformable hydrogen content has beendefined above as the number of H₂ molecules that can be derived from afuel by reforming the fuel and then driving the water gas shift reactionto completion to maximize H₂ production. For example, each methaneintroduced into an anode and exposed to steam reforming conditionsresults in generation of the equivalent of 4 H₂ molecules at maxproduction. (Depending on the reforming and/or anode conditions, thereforming product can correspond to a non-water gas shifted product,where one or more of the H₂ molecules is present instead in the form ofa CO molecule.) Thus, methane is defined as having a reformable hydrogencontent of 4 H₂ molecules. As another example, under this definitionethane has a reformable hydrogen content of 7 H₂ molecules.

The utilization of fuel in the anode can also be characterized bydefining a heating value utilization based on a ratio of the LowerHeating Value of hydrogen oxidized in the anode due to the fuel cellanode reaction relative to the Lower Heating Value of all fuel deliveredto the anode and/or a reforming stage associated with the anode. The“fuel cell heating value utilization” as used herein can be computedusing the flow rates and Lower Heating Value (LHV) of the fuelcomponents entering and leaving the fuel cell anode. As such, fuel cellheating value utilization can be computed as(LHV(anode_in)−LHV(anode_out))/LHV(anode_in), where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In this definition, the LHV of a stream or flow may be computed as a sumof values for each fuel component in the input and/or output stream. Thecontribution of each fuel component to the sum can correspond to thefuel component's flow rate (e.g., mol/hr) multiplied by the fuelcomponent's LHV (e.g., joules/mol).

Lower Heating Value: The lower heating value is defined as the enthalpyof combustion of a fuel component to vapor phase, fully oxidizedproducts (i.e., vapor phase CO₂ and H₂O product). For example, any CO₂present in an anode input stream does not contribute to the fuel contentof the anode input, since CO₂ is already fully oxidized. For thisdefinition, the amount of oxidation occurring in the anode due to theanode fuel cell reaction is defined as oxidation of H₂ in the anode aspart of the electrochemical reaction in the anode, as defined above.

It is noted that, for the special case where the only fuel in the anodeinput flow is H₂, the only reaction involving a fuel component that cantake place in the anode represents the conversion of H₂ into H₂O. Inthis special case, the fuel utilization simplifies to (H₂-rate-in minusH₂-rate-out)/H₂-rate-in. In such a case, H₂ would be the only fuelcomponent, and so the H₂ LHV would cancel out of the equation. In themore general case, the anode feed may contain, for example, CH₄, H₂, andCO in various amounts. Because these species can typically be present indifferent amounts in the anode outlet, the summation as described abovecan be needed to determine the fuel utilization.

Alternatively or in addition to fuel utilization, the utilization forother reactants in the fuel cell can be characterized. For example, theoperation of a fuel cell can additionally or alternately becharacterized with regard to “CO₂ utilization” and/or “oxidant”utilization. The values for CO₂ utilization and/or oxidant utilizationcan be specified in a similar manner.

Fuel surplus ratio: Still another way to characterize the reactions in amolten carbonate fuel cell is by defining a utilization based on a ratioof the Lower Heating Value of all fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. This quantity will be referred to as a fuel surplus ratio. Assuch the fuel surplus ratio can be computed as (LHV(anode_in)/(LHV(anode_in)−LHV(anode_out)) where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In various aspects of the invention, a molten carbonate fuel cell can beoperated to have a fuel surplus ratio of at least about 1.0, such as atleast about 1.5, or at least about 2.0, or at least about 2.5, or atleast about 3.0, or at least about 4.0. Additionally or alternately, thefuel surplus ratio can be about 25.0 or less.

It is noted that not all of the reformable fuel in the input stream forthe anode may be reformed. Preferably, at least about 90% of thereformable fuel in the input stream to the anode (and/or into anassociated reforming stage) can be reformed prior to exiting the anode,such as at least about 95% or at least about 98%. In some alternativeaspects, the amount of reformable fuel that is reformed can be fromabout 75% to about 90%, such as at least about 80%.

The above definition for fuel surplus ratio provides a method forcharacterizing the amount of reforming occurring within the anode and/orreforming stage(s) associated with a fuel cell relative to the amount offuel consumed in the fuel cell anode for generation of electric power.

Optionally, the fuel surplus ratio can be modified to account forsituations where fuel is recycled from the anode output to the anodeinput. When fuel (such as H₂, CO, and/or unreformed or partiallyreformed hydrocarbons) is recycled from anode output to anode input,such recycled fuel components do not represent a surplus amount ofreformable or reformed fuel that can be used for other purposes.Instead, such recycled fuel components merely indicate a desire toreduce fuel utilization in a fuel cell.

Reformable fuel surplus ratio: Calculating a reformable fuel surplusratio is one option to account for such recycled fuel components is tonarrow the definition of surplus fuel, so that only the LHV ofreformable fuels is included in the input stream to the anode. As usedherein the “reformable fuel surplus ratio” is defined as the LowerHeating Value of reformable fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. Under the definition for reformable fuel surplus ratio, theLHV of any H₂ or CO in the anode input is excluded. Such an LHV ofreformable fuel can still be measured by characterizing the actualcomposition entering a fuel cell anode, so no distinction betweenrecycled components and fresh components needs to be made. Although somenon-reformed or partially reformed fuel may also be recycled, in mostaspects the majority of the fuel recycled to the anode can correspond toreformed products such as H₂ or CO. Expressed mathematically, thereformable fuel surplus ratio (R_(RFS))=LHV_(RF)/LHV_(OH), whereLHV_(RF) is the Lower Heating Value (LHV) of the reformable fuel andLHV_(OH) is the Lower Heating Value (LHV) of the hydrogen oxidized inthe anode. The LHV of the hydrogen oxidized in the anode may becalculated by subtracting the LHV of the anode outlet stream from theLHV of the anode inlet stream (e.g., LHV(anode_in)−LHV(anode_out)). Invarious aspects of the invention, a molten carbonate fuel cell can beoperated to have a reformable fuel surplus ratio of at least about 0.25,such as at least about 0.5, or at least about 1.0, or at least about1.5, or at least about 2.0, or at least about 2.5, or at least about3.0, or at least about 4.0. Additionally or alternately, the reformablefuel surplus ratio can be about 25.0 or less. It is noted that thisnarrower definition based on the amount of reformable fuel delivered tothe anode relative to the amount of oxidation in the anode candistinguish between two types of fuel cell operation methods that havelow fuel utilization. Some fuel cells achieve low fuel utilization byrecycling a substantial portion of the anode output back to the anodeinput. This recycle can allow any hydrogen in the anode input to be usedagain as an input to the anode. This can reduce the amount of reforming,as even though the fuel utilization is low for a single pass through thefuel cell, at least a portion of the unused fuel is recycled for use ina later pass. Thus, fuel cells with a wide variety of fuel utilizationvalues may have the same ratio of reformable fuel delivered to the anodereforming stage(s) versus hydrogen oxidized in the anode reaction. Inorder to change the ratio of reformable fuel delivered to the anodereforming stages relative to the amount of oxidation in the anode,either an anode feed with a native content of non-reformable fuel needsto be identified, or unused fuel in the anode output needs to bewithdrawn for other uses, or both.

Reformable hydrogen surplus ratio: Still another option forcharacterizing the operation of a fuel cell is based on a “reformablehydrogen surplus ratio.” The reformable fuel surplus ratio defined aboveis defined based on the lower heating value of reformable fuelcomponents. The reformable hydrogen surplus ratio is defined as thereformable hydrogen content of reformable fuel delivered to the anodeand/or a reforming stage associated with the anode relative to thehydrogen reacted in the anode due to the fuel cell anode reaction. Assuch, the “reformable hydrogen surplus ratio” can be computed as(RFC(reformable_anode_in)/(RFC(reformable_anode_in)−RFC(anode_out)),where RFC(reformable_anode_in) refers to the reformable hydrogen contentof reformable fuels in the anode inlet streams or flows, while RFC(anode_out) refers to the reformable hydrogen content of the fuelcomponents (such as H₂, CH₄, and/or CO) in the anode inlet and outletstreams or flows. The RFC can be expressed in moles/s, moles/hr, orsimilar. An example of a method for operating a fuel cell with a largeratio of reformable fuel delivered to the anode reforming stage(s)versus amount of oxidation in the anode can be a method where excessreforming is performed in order to balance the generation andconsumption of heat in the fuel cell. Reforming a reformable fuel toform H₂ and CO is an endothermic process. This endothermic reaction canbe countered by the generation of electrical current in the fuel cell,which can also produce excess heat corresponding (roughly) to thedifference between the amount of heat generated by the anode oxidationreaction and the carbonate formation reaction and the energy that exitsthe fuel cell in the form of electric current. The excess heat per moleof hydrogen involved in the anode oxidation reaction/carbonate formationreaction can be greater than the heat absorbed to generate a mole ofhydrogen by reforming. As a result, a fuel cell operated underconventional conditions can exhibit a temperature increase from inlet tooutlet. Instead of this type of conventional operation, the amount offuel reformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can be (roughly)balanced by the heat consumed in reforming, or even the heat consumed byreforming can be beyond the excess heat generated by the fuel oxidation,resulting in a temperature drop across the fuel cell. This can result ina substantial excess of hydrogen relative to the amount needed forelectrical power generation. As one example, a feed to the anode inletof a fuel cell or an associated reforming stage can be substantiallycomposed of reformable a) fuel, such as a substantially pure methanefeed. During conventional operation for electric power generation usingsuch a fuel, a molten carbonate fuel cell can be operated with a fuelutilization of about 75%. This means that about 75% (or ¾) of the fuelcontent delivered to the anode is used to form hydrogen that is thenreacted in the anode with carbonate ions to form H₂O and CO₂. Inconventional operation, the remaining about 25% of the fuel content canbe reformed to H₂ within the fuel cell (or can pass through the fuelcell unreacted for any CO or H₂ in the fuel), and then combusted outsideof the fuel cell to form H₂O and CO₂ to provide heat for the cathodeinlet to the fuel cell. The reformable hydrogen surplus ratio in thissituation can be 4/(4−1)=4/3.

Electrical efficiency: As used herein, the term “electrical efficiency”(“EE”) is defined as the electrochemical power produced by the fuel celldivided by the rate of Lower Heating Value (“LHV”) of fuel input to thefuel cell. The fuel inputs to the fuel cell includes both fuel deliveredto the anode as well as any fuel used to maintain the temperature of thefuel cell, such as fuel delivered to a burner associated with a fuelcell. In this description, the power produced by the fuel may bedescribed in terms of LHV(el) fuel rate.

Electrochemical power: As used herein, the term “electrochemical power”or LHV(el) is the power generated by the circuit connecting the cathodeto the anode in the fuel cell and the transfer of carbonate ions acrossthe fuel cell's electrolyte. Electrochemical power excludes powerproduced or consumed by equipment upstream or downstream from the fuelcell. For example, electricity produced from heat in a fuel cell exhauststream is not considered part of the electrochemical power. Similarly,power generated by a gas turbine or other equipment upstream of the fuelcell is not part of the electrochemical power generated. The“electrochemical power” does not take electrical power consumed duringoperation of the fuel cell into account, or any loss incurred byconversion of the direct current to alternating current. In other words,electrical power used to supply the fuel cell operation or otherwiseoperate the fuel cell is not subtracted from the direct current powerproduced by the fuel cell. As used herein, the power density is thecurrent density multiplied by voltage. As used herein, the total fuelcell power is the power density multiplied by the fuel cell area.

Fuel inputs: As used herein, the term “anode fuel input,” designated asLHV(anode_in), is the amount of fuel within the anode inlet stream. Theterm “fuel input”, designated as LHV(in), is the total amount of fueldelivered to the fuel cell, including both the amount of fuel within theanode inlet stream and the amount of fuel used to maintain thetemperature of the fuel cell. The fuel may include both reformable andnonreformable fuels, based on the definition of a reformable fuelprovided herein. Fuel input is not the same as fuel utilization.

Total fuel cell efficiency: As used herein, the term “total fuel cellefficiency” (“TFCE”) is defined as: the electrochemical power generatedby the fuel cell, plus the rate of LHV of syngas produced by the fuelcell, divided by the rate of LHV of fuel input to the anode. In otherwords, TFCE=(LHV(el)+LHV(sg net))/LHV(anode_in), where LHV(anode_in)refers to rate at which the LHV of the fuel components (such as H₂, CH₄,and/or CO) delivered to the anode and LHV(sg net) refers to a rate atwhich syngas (H₂, CO) is produced in the anode, which is the differencebetween syngas input to the anode and syngas output from the anode.LHV(el) describes the electrochemical power generation of the fuel cell.The total fuel cell efficiency excludes heat generated by the fuel cellthat is put to beneficial use outside of the fuel cell. In operation,heat generated by the fuel cell may be put to beneficial use bydownstream equipment. For example, the heat may be used to generateadditional electricity or to heat water. These uses, when they occurapart from the fuel cell, are not part of the total fuel cellefficiency, as the term is used in this application. The total fuel cellefficiency is for the fuel cell operation only, and does not includepower production, or consumption, upstream, or downstream, of the fuelcell.

Chemical efficiency: As used herein, the term “chemical efficiency”, isdefined as the lower heating value of H₂ and CO in the anode exhaust ofthe fuel cell, or LHV(sg out), divided by the fuel input, or LHV(in).

Neither the electrical efficiency nor the total system efficiency takesthe efficiency of upstream or downstream processes into consideration.For example, it may be advantageous to use turbine exhaust as a sourceof CO₂ for the fuel cell cathode. In this arrangement, the efficiency ofthe turbine is not considered as part of the electrical efficiency orthe total fuel cell efficiency calculation. Similarly, outputs from thefuel cell may be recycled as inputs to the fuel cell. A recycle loop isnot considered when calculating electrical efficiency or the total fuelcell efficiency in single pass mode.

Syngas produced: As used herein, the term “syngas produced” is thedifference between syngas input to the anode and syngas output from theanode. Syngas may be used as an input, or fuel, for the anode, at leastin part. For example, a system may include an anode recycle loop thatreturns syngas from the anode exhaust to the anode inlet where it issupplemented with natural gas or other suitable fuel. Syngas producedLHV (sg net)=(LHV(sg out)−LHV(sg in)), where LHV(sg in) and LHV(sg out)refer to the LHV of the syngas in the anode inlet and syngas in theanode outlet streams or flows, respectively. It is noted that at least aportion of the syngas produced by the reforming reactions within ananode can typically be utilized in the anode to produce electricity. Thehydrogen utilized to produce electricity is not included in thedefinition of “syngas produced” because it does not exit the anode. Asused herein, the term “syngas ratio” is the LHV of the net syngasproduced divided by the LHV of the fuel input to the anode or LHV (sgnet)/LHV(anode in). Molar flow rates of syngas and fuel can be usedinstead of LHV to express a molar-based syngas ratio and a molar-basedsyngas produced.

Steam to carbon ratio (S/C): As used herein, the steam to carbon ratio(S/C) is the molar ratio of steam in a flow to reformable carbon in theflow. Carbon in the form of CO and CO₂ are not included as reformablecarbon in this definition. The steam to carbon ratio can be measuredand/or controlled at different points in the system. For example, thecomposition of an anode inlet stream can be manipulated to achieve a S/Cthat is suitable for reforming in the anode. The S/C can be given as themolar flow rate of H₂O divided by the product of the molar flow rate offuel multiplied by the number of carbon atoms in the fuel, e.g. one formethane. Thus, S/C=f_(H20)/(f_(CH4) X #C), where f_(H20) is the molarflow rate of water, where f_(CH4) is the molar flow rate of methane (orother fuel) and #C is the number of carbons in the fuel.

EGR ratio: Aspects of the invention can use a turbine in partnershipwith a fuel cell. The combined fuel cell and turbine system may includeexhaust gas recycle (“EGR”). In an EGR system, at least a portion of theexhaust gas generated by the turbine can be sent to a heat recoverygenerator. Another portion of the exhaust gas can be sent to the fuelcell. The EGR ratio describes the amount of exhaust gas routed to thefuel cell versus the total exhaust gas routed to either the fuel cell orheat recovery generator. As used herein, the “EGR ratio” is the flowrate for the fuel cell bound portion of the exhaust gas divided by thecombined flow rate for the fuel cell bound portion and the recoverybound portion, which is sent to the heat recovery generator.

In various aspects of the invention, a molten carbonate fuel cell (MCFC)can be used to facilitate separation of CO₂ from a CO₂-containing streamwhile also generating additional electrical power. The CO₂ separationcan be further enhanced by taking advantage of synergies with thecombustion-based power generator that can provide at least a portion ofthe input feed to the cathode portion of the fuel cell.

Fuel Cell and Fuel Cell Components: In this discussion, a fuel cell cancorrespond to a single cell, with an anode and a cathode separated by anelectrolyte. The anode and cathode can receive input gas flows tofacilitate the respective anode and cathode reactions for transportingcharge across the electrolyte and generating electricity. A fuel cellstack can represent a plurality of cells in an integrated unit. Althougha fuel cell stack can include multiple fuel cells, the fuel cells cantypically be connected in parallel and can function (approximately) asif they collectively represented a single fuel cell of a larger size.When an input flow is delivered to the anode or cathode of a fuel cellstack, the fuel stack can include flow channels for dividing the inputflow between each of the cells in the stack and flow channels forcombining the output flows from the individual cells. In thisdiscussion, a fuel cell array can be used to refer to a plurality offuel cells (such as a plurality of fuel cell stacks) that are arrangedin series, in parallel, or in any other convenient manner (e.g., in acombination of series and parallel). A fuel cell array can include oneor more stages of fuel cells and/or fuel cell stacks, where theanode/cathode output from a first stage may serve as the anode/cathodeinput for a second stage. It is noted that the anodes in a fuel cellarray do not have to be connected in the same way as the cathodes in thearray. For convenience, the input to the first anode stage of a fuelcell array may be referred to as the anode input for the array, and theinput to the first cathode stage of the fuel cell array may be referredto as the cathode input to the array. Similarly, the output from thefinal anode/cathode stage may be referred to as the anode/cathode outputfrom the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack”. This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuel cellelements. These individual fuel cell elements can typically have similarvoltages (as the reactant and product concentrations are similar), andthe total power output can result from the summation of all of theelectrical currents in all of the cell elements, when the elements areelectrically connected in series. Stacks can also be arranged in aseries arrangement to produce high voltages. A parallel arrangement canboost the current. If a sufficiently large volume fuel cell stack isavailable to process a given exhaust flow, the systems and methodsdescribed herein can be used with a single molten carbonate fuel cellstack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

“Internal reforming” and “external reforming”: A fuel cell or fuel cellstack may include one or more internal reforming sections. As usedherein, the term “internal reforming” refers to fuel reforming occurringwithin the body of a fuel cell, a fuel cell stack, or otherwise within afuel cell assembly. External reforming, which is often used inconjunction with a fuel cell, occurs in a separate piece of equipmentthat is located outside of the fuel cell stack. In other words, the bodyof the external reformer is not in direct physical contact with the bodyof a fuel cell or fuel cell stack. In a typical set up, the output fromthe external reformer can be fed to the anode inlet of a fuel cell.Unless otherwise noted specifically, the reforming described within thisapplication is internal reforming.

Internal reforming may occur within a fuel cell anode. Internalreforming can additionally or alternately occur within an internalreforming element integrated within a fuel cell assembly. The integratedreforming element may be located between fuel cell elements within afuel cell stack. In other words, one of the trays in the stack can be areforming section instead of a fuel cell element. In one aspect, theflow arrangement within a fuel cell stack directs fuel to the internalreforming elements and then into the anode portion of the fuel cells.Thus, from a flow perspective, the internal reforming elements and fuelcell elements can be arranged in series within the fuel cell stack. Asused herein, the term “anode reforming” is fuel reforming that occurswithin an anode. As used herein, the term “internal reforming” isreforming that occurs within an integrated reforming element and not inan anode section.

In some aspects, a reforming stage that is internal to a fuel cellassembly can be considered to be associated with the anode(s) in thefuel cell assembly. In some alternative aspects, for a reforming stagein a fuel cell stack that can be associated with an anode (such asassociated with multiple anodes), a flow path can be available so thatthe output flow from the reforming stage is passed into at least oneanode. This can correspond to having an initial section of a fuel cellplate not in contact with the electrolyte and instead can serve just asa reforming catalyst. Another option for an associated reforming stagecan be to have a separate integrated reforming stage as one of theelements in a fuel cell stack, where the output from the integratedreforming stage can be returned to the input side of one or more of thefuel cells in the fuel cell stack.

From a heat integration standpoint, a characteristic height in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage and/or a separateendothermic reaction stage could have a different height in the stackthan a fuel cell. In such a scenario, the height of a fuel cell elementcan be used as the characteristic height. In some aspects, an integratedendothermic reaction stage can be defined as a stage that is heatintegrated with one or more fuel cells, so that the integratedendothermic reaction stage can use the heat from the fuel cells as aheat source for the endothermic reaction. Such an integrated endothermicreaction stage can be defined as being positioned less than 5 times theheight of a stack element from any fuel cells providing heat to theintegrated stage. For example, an integrated endothermic reaction stage(such as a reforming stage) can be positioned less than 5 times theheight of a stack element from any fuel cells that are heat integrated,such as less than 3 times the height of a stack element. In thisdiscussion, an integrated reforming stage and/or integrated endothermicreaction stage that represent an adjacent stack element to a fuel cellelement can be defined as being about one stack element height or lessaway from the adjacent fuel cell element.

In some aspects, a separate reforming stage that is heat integrated witha fuel cell element can correspond to a reforming stage associated withthe fuel cell element. In such aspects, an integrated fuel cell elementcan provide at least a portion of the heat to the associated reformingstage, and the associated reforming stage can provide at least a portionof the reforming stage output to the integrated fuel cell as a fuelstream. In other aspects, a separate reforming stage can be integratedwith a fuel cell for heat transfer without being associated with thefuel cell. In this type of situation, the separate reforming stage canreceive heat from the fuel cell, but the decision can be made not to usethe output of the reforming stage as an input to the fuel cell. Instead,the decision can be made to use the output of such a reforming stage foranother purpose, such as directly adding the output to the anode exhauststream, and/or for forming a separate output stream from the fuel cellassembly.

More generally, a separate stack element in a fuel cell stack can beused to perform any convenient type of endothermic reaction that cantake advantage of the waste heat provided by integrated fuel cell stackelements. Instead of plates suitable for performing a reforming reactionon a hydrocarbon fuel stream, a separate stack element can have platessuitable for catalyzing another type of endothermic reaction. A manifoldor other arrangement of inlet conduits in the fuel cell stack can beused to provide an appropriate input flow to each stack element. Asimilar manifold or other arrangement of outlet conduits canadditionally or alternately be used to withdraw the output flows fromeach stack element. Optionally, the output flows from a endothermicreaction stage in a stack can be withdrawn from the fuel cell stackwithout having the output flow pass through a fuel cell anode. In suchan optional aspect, the products of the exothermic reaction cantherefore exit from the fuel cell stack without passing through a fuelcell anode. Examples of other types of endothermic reactions that can beperformed in stack elements in a fuel cell stack can include, withoutlimitation, ethanol dehydration to form ethylene and ethane cracking.

Recycle: As defined herein, recycle of a portion of a fuel cell output(such as an anode exhaust or a stream separated or withdrawn from ananode exhaust) to a fuel cell inlet can correspond to a direct orindirect recycle stream. A direct recycle of a stream to a fuel cellinlet is defined as recycle of the stream without passing through anintermediate process, while an indirect recycle involves recycle afterpassing a stream through one or more intermediate processes. Forexample, if the anode exhaust is passed through a CO₂ separation stageprior to recycle, this is considered an indirect recycle of the anodeexhaust. If a portion of the anode exhaust, such as an H₂ streamwithdrawn from the anode exhaust, is passed into a gasifier forconverting coal into a fuel suitable for introduction into the fuelcell, then that is also considered an indirect recycle.

Anode Inputs and Outputs

In various aspects of the invention, the MCFC array can be fed by a fuelreceived at the anode inlet that comprises, for example, both hydrogenand a hydrocarbon such as methane (or alternatively a hydrocarbonaceousor hydrocarbon-like compound that may contain heteroatoms different fromC and H). Most of the methane (or other hydrocarbonaceous orhydrocarbon-like compound) fed to the anode can typically be freshmethane. In this description, a fresh fuel such as fresh methane refersto a fuel that is not recycled from another fuel cell process. Forexample, methane recycled from the anode outlet stream back to the anodeinlet may not be considered “fresh” methane, and can instead bedescribed as reclaimed methane. The fuel source used can be shared withother components, such as a turbine that uses a portion of the fuelsource to provide a CO₂-containing stream for the cathode input. Thefuel source input can include water in a proportion to the fuelappropriate for reforming the hydrocarbon (or hydrocarbon-like) compoundin the reforming section that generates hydrogen. For example, ifmethane is the fuel input for reforming to generate H₂, the molar ratioof water to fuel can be from about one to one to about ten to one, suchas at least about two to one. A ratio of four to one or greater istypical for external reforming, but lower values can be typical forinternal reforming To the degree that H₂ is a portion of the fuelsource, in some optional aspects no additional water may be needed inthe fuel, as the oxidation of H₂ at the anode can tend to produce H₂Othat can be used for reforming the fuel. The fuel source can alsooptionally contain components incidental to the fuel source (e.g., anatural gas feed can contain some content of CO₂ as an additionalcomponent). For example, a natural gas feed can contain CO₂, N₂, and/orother inert (noble) gases as additional components. Optionally, in someaspects the fuel source may also contain CO, such as CO from a recycledportion of the anode exhaust. An additional or alternate potentialsource for CO in the fuel into a fuel cell assembly can be CO generatedby steam reforming of a hydrocarbon fuel performed on the fuel prior toentering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an input stream for the anode of a molten carbonate fuel cell.Some fuel streams can correspond to streams containing hydrocarbonsand/or hydrocarbon-like compounds that may also include heteroatomsdifferent from C and H. In this discussion, unless otherwise specified,a reference to a fuel stream containing hydrocarbons for an MCFC anodeis defined to include fuel streams containing such hydrocarbon-likecompounds. Examples of hydrocarbon (including hydrocarbon-like) fuelstreams include natural gas, streams containing C1-C4 carbon compounds(such as methane or ethane), and streams containing heavier C5+hydrocarbons (including hydrocarbon-like compounds), as well ascombinations thereof. Still other additional or alternate examples ofpotential fuel streams for use in an anode input can include biogas-typestreams, such as methane produced from natural (biological)decomposition of organic material.

In some aspects, a molten carbonate fuel cell can be used to process aninput fuel stream, such as a natural gas and/or hydrocarbon stream, witha low energy content due to the presence of diluent compounds. Forexample, some sources of methane and/or natural gas are sources that caninclude substantial amounts of either CO₂ or other inert molecules, suchas nitrogen, argon, or helium. Due to the presence of elevated amountsof CO₂ and/or inerts, the energy content of a fuel stream based on thesource can be reduced. Using a low energy content fuel for a combustionreaction (such as for powering a combustion-powered turbine) can posedifficulties. However, a molten carbonate fuel cell can generate powerbased on a low energy content fuel source with a reduced or minimalimpact on the efficiency of the fuel cell. The presence of additionalgas volume can require additional heat for raising the temperature ofthe fuel to the temperature for reforming and/or the anode reaction.Additionally, due to the equilibrium nature of the water gas shiftreaction within a fuel cell anode, the presence of additional CO₂ canhave an impact on the relative amounts of H₂ and CO present in the anodeoutput. However, the inert compounds otherwise can have only a minimaldirect impact on the reforming and anode reactions. The amount of CO₂and/or inert compounds in a fuel stream for a molten carbonate fuelcell, when present, can be at least about 1 vol %, such as at leastabout 2 vol %, or at least about 5 vol %, or at least about 10 vol %, orat least about 15 vol %, or at least about 20 vol %, or at least about25 vol %, or at least about 30 vol %, or at least about 35 vol %, or atleast about 40 vol %, or at least about 45 vol %, or at least about 50vol %, or at least about 75 vol %. Additionally or alternately, theamount of CO₂ and/or inert compounds in a fuel stream for a moltencarbonate fuel cell can be about 90 vol % or less, such as about 75 vol% or less, or about 60 vol % or less, or about 50 vol % or less, orabout 40 vol % or less, or about 35 vol % or less.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C1-C4 hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C1-C4) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an MCFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

In various aspects, the composition of the output stream from the anodecan be impacted by several factors. Factors that can influence the anodeoutput composition can include the composition of the input stream tothe anode, the amount of current generated by the fuel cell, and/or thetemperature at the exit of the anode. The temperature of at the anodeexit can be relevant due to the equilibrium nature of the water gasshift reaction. In a typical anode, at least one of the plates formingthe wall of the anode can be suitable for catalyzing the water gas shiftreaction. As a result, if a) the composition of the anode input streamis known, b) the extent of reforming of reformable fuel in the anodeinput stream is known, and c) the amount of carbonate transported fromthe cathode to anode (corresponding to the amount of electrical currentgenerated) is known, the composition of the anode output can bedetermined based on the equilibrium constant for the water gas shiftreaction.

K_(eq)=[CO₂][H₂]/[CO][H₂O]

In the above equation, K_(eq) is the equilibrium constant for thereaction at a given temperature and pressure, and [X] is the partialpressure of component X. Based on the water gas shift reaction, it canbe noted that an increased CO₂ concentration in the anode input can tendto result in additional CO formation (at the expense of H₂) while anincreased H₂O concentration can tend to result in additional H₂formation (at the expense of CO).

To determine the composition at the anode output, the composition of theanode input can be used as a starting point. This composition can thenbe modified to reflect the extent of reforming of any reformable fuelsthat can occur within the anode. Such reforming can reduce thehydrocarbon content of the anode input in exchange for increasedhydrogen and CO₂. Next, based on the amount of electrical currentgenerated, the amount of H₂ in the anode input can be reduced inexchange for additional H₂O and CO₂. This composition can then beadjusted based on the equilibrium constant for the water gas shiftreaction to determine the exit concentrations for H₂, CO, CO₂, and H₂O.

Table 1 shows the anode exhaust composition at different fuelutilizations for a typical type of fuel. The anode exhaust compositioncan reflect the combined result of the anode reforming reaction, watergas shift reaction, and the anode oxidation reaction. The outputcomposition values in Table 1 were calculated by assuming an anode inputcomposition with an about 2 to 1 ratio of steam (H₂O) to carbon(reformable fuel). The reformable fuel was assumed to be methane, whichwas assumed to be 100% reformed to hydrogen. The initial CO₂ and H₂concentrations in the anode input were assumed to be negligible, whilethe input N₂ concentration was about 0.5%. The fuel utilization U_(f)(as defined herein) was allowed to vary from about 35% to about 70% asshown in the table. The exit temperature for the fuel cell anode wasassumed to be about 650° C. for purposes of determining the correctvalue for the equilibrium constant.

TABLE 1 Anode Exhaust Composition Uf % 35% 40% 45% 50% 55% 60% 65% 70%Anode Exhaust Composition H₂O %, wet 32.5% 34.1% 35.5% 36.7% 37.8% 38.9%39.8% 40.5% CO₂ %, wet 26.7% 29.4% 32.0% 34.5% 36.9% 39.3% 41.5% 43.8%H₂ %, wet 29.4% 26.0% 22.9% 20.0% 17.3% 14.8% 12.5% 10.4% CO %, wet10.8% 10.0% 9.2% 8.4% 7.5% 6.7% 5.8% 4.9% N₂ %, wet 0.5% 0.5% 0.5% 0.4%0.4% 0.4% 0.4% 0.4% CO₂ %, dry 39.6% 44.6% 49.6% 54.5% 59.4% 64.2% 69.0%73.7% H₂ %, dry 43.6% 39.4% 35.4% 31.5% 27.8% 24.2% 20.7% 17.5% CO %,dry 16.1% 15.2% 14.3% 13.2% 12.1% 10.9% 9.7% 8.2% N₂ %, dry 0.7% 0.7%0.7% 0.7% 0.7% 0.7% 0.7% 0.7% H₂/CO 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.1(H₂—CO₂)/ 0.07 −0.09 −0.22 −0.34 −0.44 −0.53 −0.61 −0.69 (CO + CO₂)

Table 1 shows anode output compositions for a particular set ofconditions and anode input composition. More generally, in variousaspects the anode output can include about 10 vol % to about 50 vol %H₂O. The amount of H₂O can vary greatly, as H₂O in the anode can beproduced by the anode oxidation reaction. If an excess of H₂O beyondwhat is needed for reforming is introduced into the anode, the excessH₂O can typically pass through largely unreacted, with the exception ofH₂O consumed (or generated) due to fuel reforming and the water gasshift reaction. The CO₂ concentration in the anode output can also varywidely, such as from about 20 vol % to about 50 vol % CO₂. The amount ofCO₂ can be influenced by both the amount of electrical current generatedas well as the amount of CO₂ in the anode input flow. The amount of H₂in the anode output can additionally or alternately be from about 10 vol% H₂ to about 50 vol % H₂, depending on the fuel utilization in theanode. At the anode output, the amount of CO can be from about 5 vol %to about 20 vol %. It is noted that the amount of CO relative to theamount of H₂ in the anode output for a given fuel cell can be determinedin part by the equilibrium constant for the water gas shift reaction atthe temperature and pressure present in the fuel cell. The anode outputcan further additionally or alternately include 5 vol % or less ofvarious other components, such as N₂, CH₄ (or other unreactedcarbon-containing fuels), and/or other components.

Optionally, one or more water gas shift reaction stages can be includedafter the anode output to convert CO and H₂O in the anode output intoCO₂ and H₂, if desired. The amount of H₂ present in the anode output canbe increased, for example, by using a water gas shift reactor at lowertemperature to convert H₂O and CO present in the anode output into H₂and CO₂. Alternatively, the temperature can be raised and the water-gasshift reaction can be reversed, producing more CO and H₂O from H₂ andCO₂. Water is an expected output of the reaction occurring at the anode,so the anode output can typically have an excess of H₂O relative to theamount of CO present in the anode output. Alternatively, H₂O can beadded to the stream after the anode exit but before the water gas shiftreaction. CO can be present in the anode output due to incomplete carbonconversion during reforming and/or due to the equilibrium balancingreactions between H₂O, CO, H₂, and CO₂ (i.e., the water-gas shiftequilibrium) under either reforming conditions or the conditions presentduring the anode reaction. A water gas shift reactor can be operatedunder conditions to drive the equilibrium further in the direction offorming CO₂ and H₂ at the expense of CO and H₂O. Higher temperatures cantend to favor the formation of CO and H₂O. Thus, one option foroperating the water gas shift reactor can be to expose the anode outputstream to a suitable catalyst, such as a catalyst including iron oxide,zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can additionally or alternately increase theamount of CO₂ at the expense of CO. This can exchangedifficult-to-remove carbon monoxide (CO) for carbon dioxide, which canbe more readily removed by condensation (e.g., cryogenic removal),chemical reaction (such as amine removal), and/or other CO₂ removalmethods. Additionally or alternately, it may be desirable to increasethe CO content present in the anode exhaust in order to achieve adesired ratio of H₂ to CO.

After passing through the optional water gas shift reaction stage, theanode output can be passed through one or more separation stages forremoval of water and/or CO₂ from the anode output stream. For example,one or more CO₂ output streams can be formed by performing CO₂separation on the anode output using one or more methods individually orin combination. Such methods can be used to generate CO₂ outputstream(s) having a CO₂ content of 90 vol % or greater, such as at least95% vol % CO₂, or at least 98 vol % CO₂. Such methods can recover aboutat least about 70% of the CO₂ content of the anode output, such as atleast about 80% of the CO₂ content of the anode output, or at leastabout 90%. Alternatively, in some aspects it may be desirable to recoveronly a portion of the CO₂ within an anode output stream, with therecovered portion of CO₂ being about 33% to about 90% of the CO₂ in theanode output, such as at least about 40%, or at least about 50%. Forexample, it may be desirable to retain some CO₂ in the anode output flowso that a desired composition can be achieved in a subsequent water gasshift stage. Suitable separation methods may comprise use of a physicalsolvent (e.g., Selexol™ or Rectisol™); amines or other bases (e.g., MEAor MDEA); refrigeration (e.g., cryogenic separation); pressure swingadsorption; vacuum swing adsorption; and combinations thereof. Acryogenic CO₂ separator can be an example of a suitable separator. Asthe anode output is cooled, the majority of the water in the anodeoutput can be separated out as a condensed (liquid) phase. Furthercooling and/or pressurizing of the water-depleted anode output flow canthen separate high purity CO₂, as the other remaining components in theanode output flow (such as H₂, N₂, CH₄) do not tend to readily formcondensed phases. A cryogenic CO₂ separator can recover between about33% and about 90% of the CO₂ present in a flow, depending on theoperating conditions.

Removal of water from the anode exhaust to form one or more water outputstreams can also be beneficial, whether prior to, during, or afterperforming CO₂ separation. The amount of water in the anode output canvary depending on operating conditions selected. For example, thesteam-to-carbon ratio established at the anode inlet can affect thewater content in the anode exhaust, with high steam-to-carbon ratiostypically resulting in a large amount of water that can pass through theanode unreacted and/or reacted only due to the water gas shiftequilibrium in the anode. Depending on the aspect, the water content inthe anode exhaust can correspond to up to about 30% or more of thevolume in the anode exhaust. Additionally or alternately, the watercontent can be about 80% or less of the volume of the anode exhaust.While such water can be removed by compression and/or cooling withresulting condensation, the removal of this water can require extracompressor power and/or heat exchange surface area and excessive coolingwater. One beneficial way to remove a portion of this excess water canbe based on use of an adsorbent bed that can capture the humidity fromthe moist anode effluent and can then be ‘regenerated’ using dry anodefeed gas, in order to provide additional water for the anode feed.HVAC-style (heating, ventilation, and air conditioning) adsorptionwheels design can be applicable, because anode exhaust and inlet can besimilar in pressure, and minor leakage from one stream to the other canhave minimal impact on the overall process. In embodiments where CO₂removal is performed using a cryogenic process, removal of water priorto or during CO₂ removal may be desirable, including removal bytriethyleneglycol (TEG) system and/or desiccants. By contrast, if anamine wash is used for CO₂ removal, water can be removed from the anodeexhaust downstream from the CO₂ removal stage.

Alternately or in addition to a CO₂ output stream and/or a water outputstream, the anode output can be used to form one or more product streamscontaining a desired chemical or fuel product. Such a product stream orstreams can correspond to a syngas stream, a hydrogen stream, or bothsyngas product and hydrogen product streams. For example, a hydrogenproduct stream containing at least about 70 vol % H₂, such as at leastabout 90 vol % H₂ or at least about 95 vol % H₂, can be formed.Additionally or alternately, a syngas stream containing at least about70 vol % of H₂ and CO combined, such as at least about 90 vol % of H₂and CO can be formed. The one or more product streams can have a gasvolume corresponding to at least about 75% of the combined H₂ and CO gasvolumes in the anode output, such as at least about 85% or at leastabout 90% of the combined H₂ and CO gas volumes. It is noted that therelative amounts of H₂ and CO in the products streams may differ fromthe H₂ to CO ratio in the anode output based on use of water gas shiftreaction stages to convert between the products.

In some aspects, it can be desirable to remove or separate a portion ofthe H₂ present in the anode output. For example, in some aspects the H₂to CO ratio in the anode exhaust can be at least about 3.0:1. Bycontrast, processes that make use of syngas, such as Fischer-Tropschsynthesis, may consume H₂ and CO in a different ratio, such as a ratiothat is closer to 2:1. One alternative can be to use a water gas shiftreaction to modify the content of the anode output to have an H₂ to COratio closer to a desired syngas composition. Another alternative can beto use a membrane separation to remove a portion of the H₂ present inthe anode output to achieve a desired ratio of H₂ and CO, or stillalternately to use a combination of membrane separation and water gasshift reactions. One advantage of using a membrane separation to removeonly a portion of the H₂ in the anode output can be that the desiredseparation can be performed under relatively mild conditions. Since onegoal can be to produce a retentate that still has a substantial H₂content, a permeate of high purity hydrogen can be generated by membraneseparation without requiring severe conditions. For example, rather thanhaving a pressure on the permeate side of the membrane of about 100 kPaaor less (such as ambient pressure), the permeate side can be at anelevated pressure relative to ambient while still having sufficientdriving force to perform the membrane separation. Additionally oralternately, a sweep gas such as methane can be used to provide adriving force for the membrane separation. This can reduce the purity ofthe H₂ permeate stream, but may be advantageous, depending on thedesired use for the permeate stream.

In various aspects of the invention, at least a portion of the anodeexhaust stream (preferably after separation of CO₂ and/or H₂O) can beused as a feed for a process external to the fuel cell and associatedreforming stages. In various aspects, the anode exhaust can have a ratioof H₂ to CO of about 1.5:1 to about 10:1, such as at least about 3.0:1,or at least about 4.0:1, or at least about 5.0:1. A syngas stream can begenerated or withdrawn from the anode exhaust. The anode exhaust gas,optionally after separation of CO₂ and/or H₂O, and optionally afterperforming a water gas shift reaction and/or a membrane separation toremove excess hydrogen, can correspond to a stream containingsubstantial portions of H₂ and/or CO. For a stream with a relatively lowcontent of CO, such as a stream where the ratio of H₂ to CO is at leastabout 3:1, the anode exhaust can be suitable for use as an H₂ feed.Examples of processes that could benefit from an H₂ feed can include,but are not limited to, refinery processes, an ammonia synthesis plant,or a turbine in a (different) power generation system, or combinationsthereof. Depending on the application, still lower CO₂ contents can bedesirable. For a stream with an H₂-to-CO ratio of less than about 2.2 to1 and greater than about 1.9 to 1, the stream can be suitable for use asa syngas feed. Examples of processes that could benefit from a syngasfeed can include, but are not limited to, a gas-to-liquids plant (suchas a plant using a Fischer-Tropsch process with a non-shifting catalyst)and/or a methanol synthesis plant. The amount of the anode exhaust usedas a feed for an external process can be any convenient amount.Optionally, when a portion of the anode exhaust is used as a feed for anexternal process, a second portion of the anode exhaust can be recycledto the anode input and/or recycled to the combustion zone for acombustion-powered generator.

The input streams useful for different types of Fischer-Tropschsynthesis processes can provide an example of the different types ofproduct streams that may be desirable to generate from the anode output.For a Fischer-Tropsch synthesis reaction system that uses a shiftingcatalyst, such as an iron-based catalyst, the desired input stream tothe reaction system can include CO₂ in addition to H₂ and CO. If asufficient amount of CO₂ is not present in the input stream, aFischer-Tropsch catalyst with water gas shift activity can consume CO inorder to generate additional CO₂, resulting in a syngas that can bedeficient in CO. For integration of such a Fischer-Tropsch process withan MCFC fuel cell, the separation stages for the anode output can beoperated to retain a desired amount of CO₂ (and optionally H₂O) in thesyngas product. By contrast, for a Fischer-Tropsch catalyst based on anon-shifting catalyst, any CO₂ present in a product stream could serveas an inert component in the Fischer-Tropsch reaction system.

In an aspect where the membrane is swept with a sweep gas such as amethane sweep gas, the methane sweep gas can correspond to a methanestream used as the anode fuel or in a different low pressure process,such as a boiler, furnace, gas turbine, or other fuel-consuming device.In such an aspect, low levels of CO₂ permeation across the membrane canhave minimal consequence. Such CO₂ that may permeate across the membranecan have a minimal impact on the reactions within the anode, and suchCO₂ can remain contained in the anode product. Therefore, the CO₂ (ifany) lost across the membrane due to permeation does not need to betransferred again across the MCFC electrolyte. This can significantlyreduce the separation selectivity requirement for the hydrogenpermeation membrane. This can allow, for example, use of ahigher-permeability membrane having a lower selectivity, which canenable use of a lower pressure and/or reduced membrane surface area. Insuch an aspect of the invention, the volume of the sweep gas can be alarge multiple of the volume of hydrogen in the anode exhaust, which canallow the effective hydrogen concentration on the permeate side to bemaintained close to zero. The hydrogen thus separated can beincorporated into the turbine-fed methane where it can enhance theturbine combustion characteristics, as described above.

It is noted that excess H₂ produced in the anode can represent a fuelwhere the greenhouse gases have already been separated. Any CO₂ in theanode output can be readily separated from the anode output, such as byusing an amine wash, a cryogenic CO₂ separator, and/or a pressure orvacuum swing absorption process. Several of the components of the anodeoutput (H₂, CO, CH₄) are not easily removed, while CO₂ and H₂O canusually be readily removed. Depending on the embodiment, at least about90 vol % of the CO₂ in the anode output can be separated out to form arelatively high purity CO₂ output stream. Thus, any CO₂ generated in theanode can be efficiently separated out to form a high purity CO₂ outputstream. After separation, the remaining portion of the anode output cancorrespond primarily to components with chemical and/or fuel value, aswell as reduced amounts of CO₂ and/or H₂O, Since a substantial portionof the CO₂ generated by the original fuel (prior to reforming) can havebeen separated out, the amount of CO₂ generated by subsequent burning ofthe remaining portion of the anode output can be reduced. In particular,to the degree that the fuel in the remaining portion of the anode outputis H₂, no additional greenhouse gases can typically be formed by burningof this fuel.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 6and 7.

FIG. 6 schematically shows an example of a reaction system for operatinga fuel cell array of molten carbonate fuel cells in conjunction with achemical synthesis process. In FIG. 6, a fuel stream 605 is provided toa reforming stage (or stages) 610 associated with the anode 627 of afuel cell 620, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 610 associated with fuel cell 620can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 605 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 605 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 685. It is noted that anode recycle stream 685 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 625 back to anode 627, either directly or indirectly viacombination with fuel stream 605 or reformed fuel stream 615. Afterreforming, the reformed fuel stream 615 can be passed into anode 627 offuel cell 620. A CO₂ and O₂-containing stream 619 can also be passedinto cathode 629. A flow of carbonate ions 622, CO₃ ²⁻, from the cathodeportion 629 of the fuel cell can provide the remaining reactant neededfor the anode fuel cell reactions. Based on the reactions in the anode627, the resulting anode exhaust 625 can include H₂O, CO₂, one or morecomponents corresponding to incompletely reacted fuel (H₂, CO, CH₄, orother components corresponding to a reformable fuel), and optionally oneor more additional nonreactive components, such as N₂ and/or othercontaminants that are part of fuel stream 605. The anode exhaust 625 canthen be passed into one or more separation stages. For example, a CO₂removal stage 640 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream643 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 630 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 635. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 650 maybe desirable to remove a water output stream 653 from the anode exhaust.Though shown in FIG. 6 after the CO₂ separation stage 640, it mayoptionally be located before the CO₂ separation stage 640 instead.Additionally, an optional membrane separation stage 660 for separationof H₂ can be used to generate a high purity permeate stream 663 of H₂.The resulting retentate stream 666 can then be used as an input to achemical synthesis process. Stream 666 could additionally or alternatelybe shifted in a second water-gas shift reactor 631 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 668 forfurther use in a chemical synthesis process. In FIG. 6, anode recyclestream 685 is shown as being withdrawn from the retentate stream 666,but the anode recycle stream 685 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content ofCO₂ 639 can be generated as an output from cathode 629. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 7 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 7, anode exhaust 625 can be initially passed intoseparation stage 760 for removing a portion 763 of the hydrogen contentfrom the anode exhaust 625. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 766 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage730. The water gas shifted output 735 can then pass through CO₂separation stage 740 and water removal stage 750 to produce an outputstream 775 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 775 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 775 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

Conventionally, a molten carbonate fuel cell can be operated based ondrawing a desired load while consuming some portion of the fuel in thefuel stream delivered to the anode. The voltage of the fuel cell canthen be determined by the load, fuel input to the anode, air and CO₂provided to the cathode, and the internal resistances of the fuel cell.The CO₂ to the cathode can be conventionally provided in part by usingthe anode a) exhaust as at least a part of the cathode input stream. Bycontrast, the present invention can use separate/different sources forthe anode input and cathode input. By removing any direct link betweenthe composition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell, such asto generate excess synthesis gas, to improve capture of carbon dioxide,and/or to improve the total efficiency (electrical plus chemical power)of the fuel cell, among others.

In a molten carbonate fuel cell, the transport of carbonate ions acrossthe electrolyte in the fuel cell can provide a method for transportingCO₂ from a first flow path to a second flow path, where the transportmethod can allow transport from a lower concentration (the cathode) to ahigher concentration (the anode), which can thus facilitate capture ofCO₂. Part of the selectivity of the fuel cell for CO₂ separation can bebased on the electrochemical reactions allowing the cell to generateelectrical power. For nonreactive species (such as N₂) that effectivelydo not participate in the electrochemical reactions within the fuelcell, there can be an insignificant amount of reaction and transportfrom cathode to anode. By contrast, the potential (voltage) differencebetween the cathode and anode can provide a strong driving force fortransport of carbonate ions across the fuel cell. As a result, thetransport of carbonate ions in the molten carbonate fuel cell can allowCO₂ to be transported from the cathode (lower CO₂ concentration) to theanode (higher CO₂ concentration) with relatively high selectivity.However, a challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. The voltageand/or power generated by a carbonate fuel cell can start to droprapidly as the CO₂ concentration falls below about 2.0 vol %. As the CO₂concentration drops further, e.g., to below about 1.0 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function. Thus, at least some CO₂ is likely to be present in theexhaust gas from the cathode stage of a fuel cell under commerciallyviable operating conditions.

The amount of carbon dioxide delivered to the fuel cell cathode(s) canbe determined based on the CO₂ content of a source for the cathodeinlet. One example of a suitable CO₂-containing stream for use as acathode input flow can be an output or exhaust flow from a combustionsource. Examples of combustion sources include, but are not limited to,sources based on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air). To a first approximation, the CO₂ content of the outputflow from a combustion source can be a minor portion of the flow. Evenfor a higher CO₂ content exhaust flow, such as the output from acoal-fired combustion source, the CO₂ content from most commercialcoal-fired power plants can be about 15 vol % or less. More generally,the CO₂ content of an output or exhaust flow from a combustion sourcecan be at least about 1.5 vol %, or at least about 1.6 vol %, or atleast about 1.7 vol %, or at least about 1.8 vol %, or at least about1.9 vol %, or at least greater 2 vol %, or at least about 4 vol %, or atleast about 5 vol %, or at least about 6 vol %, or at least about 8 vol%. Additionally or alternately, the CO₂ content of an output or exhaustflow from a combustion source can be about 20 vol % or less, such asabout 15 vol % or less, or about 12 vol % or less, or about 10 vol % orless, or about 9 vol % or less, or about 8 vol % or less, or about 7 vol% or less, or about 6.5 vol % or less, or about 6 vol % or less, orabout 5.5 vol % or less, or about 5 vol % or less, or about 4.5 vol % orless. The concentrations given above are on a dry basis. It is notedthat the lower CO₂ content values can be present in the exhaust fromsome natural gas or methane combustion sources, such as generators thatare part of a power generation system that may or may not include anexhaust gas recycle loop.

Other potential sources for a cathode input stream can additionally oralternately include sources of bio-produced CO₂. This can include, forexample, CO₂ generated during processing of bio-derived compounds, suchas CO₂ generated during ethanol production. An additional or alternateexample can include CO₂ generated by combustion of a bio-produced fuel,such as combustion of lignocellulose. Still other additional oralternate potential CO₂ sources can correspond to output or exhauststreams from various industrial processes, such as CO₂-containingstreams generated by plants for manufacture of steel, cement, and/orpaper.

Yet another additional or alternate potential source of CO₂ can beCO₂-containing streams from a fuel cell. The CO₂-containing stream froma fuel cell can correspond to a cathode output stream from a differentfuel cell, an anode output stream from a different fuel cell, a recyclestream from the cathode output to the cathode input of a fuel cell,and/or a recycle stream from an anode output to a cathode input of afuel cell. For example, an MCFC operated in standalone mode underconventional conditions can generate a cathode exhaust with a CO₂concentration of at least about 5 vol %. Such a CO₂-containing cathodeexhaust could be used as a cathode input for an MCFC operated accordingto an aspect of the invention. More generally, other types of fuel cellsthat generate a CO₂ output from the cathode exhaust can additionally oralternately be used, as well as other types of CO₂-containing streamsnot generated by a “combustion” reaction and/or by a combustion-poweredgenerator. Optionally but preferably, a CO₂-containing stream fromanother fuel cell can be from another molten carbonate fuel cell. Forexample, for molten carbonate fuel cells connected in series withrespect to the cathodes, the output from the cathode for a first moltencarbonate fuel cell can be used as the input to the cathode for a secondmolten carbonate fuel cell.

For various types of CO₂-containing streams from sources other thancombustion sources, the CO₂ content of the stream can vary widely. TheCO₂ content of an input stream to a cathode can contain at least about 2vol % of CO₂, such as at least about 4 vol %, or at least about 5 vol %,or at least about 6 vol %, or at least about 8 vol %. Additionally oralternately, the CO₂ content of an input stream to a cathode can beabout 30 vol % or less, such as about 25 vol % or less, or about 20 vol% or less, or about 15 vol % or less, or about 10 vol % or less, orabout 8 vol % or less, or about 6 vol % or less, or about 4 vol % orless. For some still higher CO₂ content streams, the CO₂ content can begreater than about 30 vol %, such as a stream substantially composed ofCO₂ with only incidental amounts of other compounds. As an example, agas-fired turbine without exhaust gas recycle can produce an exhauststream with a CO₂ content of approximately 4.2 vol %. With EGR, agas-fired turbine can produce an exhaust stream with a CO₂ content ofabout 6-8 vol %. Stoichiometric combustion of methane can produce anexhaust stream with a CO₂ content of about 11 vol %. Combustion of coalcan produce an exhaust stream with a CO₂ content of about 15-20 vol %.Fired heaters using refinery off-gas can produce an exhaust stream witha CO₂ content of about 12-15 vol %. A gas turbine operated on a low BTUgas without any EGR can produce an exhaust stream with a CO₂ content of˜12 vol %.

In addition to CO₂, a cathode input stream must include O₂ to providethe components necessary for the cathode reaction. Some cathode inputstreams can be based on having air as a component. For example, acombustion exhaust stream can be formed by combusting a hydrocarbon fuelin the presence of air. Such a combustion exhaust stream, or anothertype of cathode input stream having an oxygen content based on inclusionof air, can have an oxygen content of about 20 vol % or less, such asabout 15 vol % or less, or about 10 vol % or less. Additionally oralternately, the oxygen content of the cathode input stream can be atleast about 4 vol %, such as at least about 6 vol %, or at least about 8vol %. More generally, a cathode input stream can have a suitablecontent of oxygen for performing the cathode reaction. In some aspects,this can correspond to an oxygen content of about 5 vol % to about 15vol %, such as from about 7 vol % to about 9 vol %. For many types ofcathode input streams, the combined amount of CO₂ and O₂ can correspondto less than about 21 vol % of the input stream, such as less than about15 vol % of the stream or less than about 10 vol % of the stream. An airstream containing oxygen can be combined with a CO₂ source that has lowoxygen content. For example, the exhaust stream generated by burningcoal may include a low oxygen content that can be mixed with air to forma cathode inlet stream.

In addition to CO₂ and O₂, a cathode input stream can also be composedof inert/non-reactive species such as N₂, H₂O, and other typical oxidant(air) components. For example, for a cathode input derived from anexhaust from a combustion reaction, if air is used as part of theoxidant source for the combustion reaction, the exhaust gas can includetypical components of air such as N₂, H₂O, and other compounds in minoramounts that are present in air. Depending on the nature of the fuelsource for the combustion reaction, additional species present aftercombustion based on the fuel source may include one or more of H₂O,oxides of nitrogen (NOx) and/or sulfur (SOx), and other compounds eitherpresent in the fuel and/or that are partial or complete combustionproducts of compounds present in the fuel, such as CO. These species maybe present in amounts that do not poison the cathode catalyst surfacesthough they may reduce the overall cathode activity. Such reductions inperformance may be acceptable, or species that interact with the cathodecatalyst may be reduced to acceptable levels by known pollutant removaltechnologies.

The amount of O₂ present in a cathode input stream (such as an inputcathode stream based on a combustion exhaust) can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of CO₂ in the exhaust. Optionally, asnecessary, additional air can be added to the cathode input to providesufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂₅ or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

In addition to the reaction to form carbonate ions for transport acrossthe electrolyte, the conditions in the cathode can also be suitable forconversion of nitrogen oxides into nitrate and/or nitrate ions.Hereinafter, only nitrate ions will be referred to for convenience. Theresulting nitrate ions can also be transported across the electrolytefor reaction in the anode. NOx concentrations in a cathode input streamcan typically be on the order of ppm, so this nitrate transport reactioncan have a minimal impact on the amount of carbonate transported acrossthe electrolyte. However, this method of NOx removal can be beneficialfor cathode input streams based on combustion exhausts from gasturbines, as this can provide a mechanism for reducing NOx emissions.The conditions in the cathode can additionally or alternately besuitable for conversion of unburned hydrocarbons (in combination with O₂in the cathode input stream) to typical combustion products, such as CO₂and H₂O.

A suitable temperature for operation of an MCFC can be between about450° C. and about 750° C., such as at least about 500° C., e.g., with aninlet temperature of about 550° C. and an outlet temperature of about625° C. Prior to entering the cathode, heat can be added to or removedfrom the combustion exhaust, if desired, e.g., to provide heat for otherprocesses, such as reforming the fuel input for the anode. For example,if the source for the cathode input stream is a combustion exhauststream, the combustion exhaust stream may have a temperature greaterthan a desired temperature for the cathode inlet. In such an aspect,heat can be removed from the combustion exhaust prior to use as thecathode input stream. Alternatively, the combustion exhaust could be atvery low temperature, for example after a wet gas scrubber on acoal-fired boiler, in which case the combustion exhaust can be belowabout 100° C. Alternatively, the combustion exhaust could be from theexhaust of a gas turbine operated in combined cycle mode, in which thegas can be cooled by raising steam to run a steam turbine for additionalpower generation. In this case, the gas can be below about 50° C. Heatcan be added to a combustion exhaust that is cooler than desired.

Fuel Cell Arrangement

In various aspects, a configuration option for a fuel cell (such as afuel cell array containing multiple fuel cell stacks) can be to dividethe CO₂-containing stream between a plurality of fuel cells. Some typesof sources for CO₂-containing streams can generate large volumetric flowrates relative to the capacity of an individual fuel cell. For example,the CO₂-containing output stream from an industrial combustion sourcecan typically correspond to a large flow volume relative to desirableoperating conditions for a single MCFC of reasonable size. Instead ofprocessing the entire flow in a single MCFC, the flow can be dividedamongst a plurality of MCFC units, usually at least some of which can bein parallel, so that the flow rate in each unit can be within a desiredflow range.

A second configuration option can be to utilize fuel cells in series tosuccessively remove CO₂ from a flow stream. Regardless of the number ofinitial fuel cells to which a CO₂-containing stream can be distributedto in parallel, each initial fuel cell can be followed by one or moreadditional cells in series to further remove additional CO₂. If thedesired amount of CO₂ in the cathode output is sufficiently low,attempting to remove CO₂ from a cathode input stream down to the desiredlevel in a single fuel cell or fuel cell stage could lead to a lowand/or unpredictable voltage output for the fuel cell. Rather thanattempting to remove CO₂ to the desired level in a single fuel cell orfuel cell stage, CO₂ can be removed in successive cells until a desiredlevel can be achieved. For example, each cell in a series of fuel cellscan be used to remove some percentage (e.g., about 50%) of the CO₂present in a fuel stream. In such an example, if three fuel cells areused in series, the CO₂ concentration can be reduced (e.g., to about 15%or less of the original amount present, which can correspond to reducingthe CO₂ concentration from about 6% to about 1% or less over the courseof three fuel cells in series).

In another configuration, the operating conditions can be selected inearly fuel stages in series to provide a desired output voltage whilethe array of stages can be selected to achieve a desired level of carbonseparation. As an example, an array of fuel cells can be used with threefuel cells in series. The first two fuel cells in series can be used toremove CO₂ while maintaining a desired output voltage. The final fuelcell can then be operated to remove CO₂ to a desired concentration butat a lower voltage.

In still another configuration, there can be separate connectivity forthe anodes and cathodes in a fuel cell array. For example, if the fuelcell array includes fuel cathodes connected in series, the correspondinganodes can be connected in any convenient manner, not necessarilymatching up with the same arrangement as their corresponding cathodes,for example. This can include, for instance, connecting the anodes inparallel, so that each anode receives the same type of fuel feed, and/orconnecting the anodes in a reverse series, so that the highest fuelconcentration in the anodes can correspond to those cathodes having thelowest CO₂ concentration.

In yet another configuration, the amount of fuel delivered to one ormore anode stages and/or the amount of CO₂ delivered to one or morecathode stages can be controlled in order to improve the performance ofthe fuel cell array. For example, a fuel cell array can have a pluralityof cathode stages connected in series. In an array that includes threecathode stages in series, this can mean that the output from a firstcathode stage can correspond to the input for a second cathode stage,and the output from the second cathode stage can correspond to the inputfor a third cathode stage. In this type of configuration, the CO₂concentration can decrease with each successive cathode stage. Tocompensate for this reduced CO₂ concentration, additional hydrogenand/or methane can be delivered to the anode stages corresponding to thelater cathode stages. The additional hydrogen and/or methane in theanodes corresponding to the later cathode stages can at least partiallyoffset the loss of voltage and/or current caused by the reduced CO₂concentration, which can increase the voltage and thus net powerproduced by the fuel cell. In another example, the cathodes in a fuelcell array can be connected partially in series and partially inparallel. In this type of example, instead of passing the entirecombustion output into the cathodes in the first cathode stage, at leasta portion of the combustion exhaust can be passed into a later cathodestage. This can provide an increased CO₂ content in a later cathodestage. Still other options for using variable feeds to either anodestages or cathode stages can be used if desired.

The cathode of a fuel cell can correspond to a plurality of cathodesfrom an array of fuel cells, as previously described. In some aspects, afuel cell array can be operated to improve or maximize the amount ofcarbon transferred from the cathode to the anode. In such aspects, forthe cathode output from the final cathode(s) in an array sequence(typically at least including a series arrangement, or else the finalcathode(s) and the initial cathode(s) would be the same), the outputcomposition can include about 2.0 vol % or less of CO₂ (e.g., about 1.5vol % or less or about 1.2 vol % or less) and/or at least about 1.0 vol% of CO₂, such as at least about 1.2 vol % or at least about 1.5 vol %.Due to this limitation, the net efficiency of CO₂ removal when usingmolten carbonate fuel cells can be dependent on the amount of CO₂ in thecathode input. For cathode input streams with CO₂ contents of greaterthan about 6 vol %, such as at least about 8%, the limitation on theamount of CO₂ that can be removed is not severe. However, for acombustion reaction using natural gas as a fuel and with excess air, asis typically found in a gas turbine, the amount of CO₂ in the combustionexhaust may only correspond to a CO₂ concentration at the cathode inputof less than about 5 vol %. Use of exhaust gas recycle can allow theamount of CO₂ at the cathode input to be increased to at least about 5vol %, e.g., at least about 6 vol %. If EGR is increased when usingnatural gas as a fuel to produce a CO₂ concentration beyond about 6 vol%, then the flammability in the combustor can be decreased and the gasturbine may become unstable. However, when H₂ is added to the fuel, theflammability window can be significantly increased, allowing the amountof exhaust gas recycle to be increased further, so that concentrationsof CO₂ at the cathode input of at least about 7.5 vol % or at leastabout 8 vol % can be achieved. As an example, based on a removal limitof about 1.5 vol % at the cathode exhaust, increasing the CO₂ content atthe cathode input from about 5.5 vol % to about 7.5 vol % can correspondto a ˜10% increase in the amount of CO₂ that can be captured using afuel cell and transported to the anode loop for eventual CO₂ separation.The amount of O₂ in the cathode output can additionally or alternatelybe reduced, typically in an amount proportional to the amount of CO₂removed, which can result in small corresponding increases in theamount(s) of the other (non-cathode-reactive) species at the cathodeexit.

In other aspects, a fuel cell array can be operated to improve ormaximize the energy output of the fuel cell, such as the total energyoutput, the electric energy output, the syngas chemical energy output,or a combination thereof. For example, molten carbonate fuel cells canbe operated with an excess of reformable fuel in a variety ofsituations, such as for generation of a syngas stream for use inchemical synthesis plant and/or for generation of a high purity hydrogenstream. The syngas stream and/or hydrogen stream can be used as a syngassource, a hydrogen source, as a clean fuel source, and/or for any otherconvenient application. In such aspects, the amount of CO₂ in thecathode exhaust can be related to the amount of CO₂ in the cathode inputstream and the CO₂ utilization at the desired operating conditions forimproving or maximizing the fuel cell energy output.

Additionally or alternately, depending on the operating conditions, anMCFC can lower the CO₂ content of a cathode exhaust stream to about 5.0vol % or less, e.g., about 4.0 vol % or less, or about 2.0 vol % orless, or about 1.5 vol % or less, or about 1.2 vol % or less.Additionally or alternately, the CO₂ content of the cathode exhauststream can be at least about 0.9 vol %, such as at least about 1.0 vol%, or at least about 1.2 vol %, or at least about 1.5 vol %.

Molten Carbonate Fuel Cell Operation

In some aspects, a fuel cell may be operated in a single pass oronce-through mode. In single pass mode, reformed products in the anodeexhaust are not returned to the anode inlet. Thus, recycling syngas,hydrogen, or some other product from the anode output directly to theanode inlet is not done in single pass operation. More generally, insingle pass operation, reformed products in the anode exhaust are alsonot returned indirectly to the anode inlet, such as by using reformedproducts to process a fuel stream subsequently introduced into the anodeinlet. Optionally, CO₂ from the anode outlet can be recycled to thecathode inlet during operation of an MCFC in single pass mode. Moregenerally, in some alternative aspects, recycling from the anode outletto the cathode inlet may occur for an MCFC operating in single passmode. Heat from the anode exhaust or output may additionally oralternately be recycled in a single pass mode. For example, the anodeoutput flow may pass through a heat exchanger that cools the anodeoutput and warms another stream, such as an input stream for the anodeand/or the cathode. Recycling heat from anode to the fuel cell isconsistent with use in single pass or once-through operation. Optionallybut not preferably, constituents of the anode output may be burned toprovide heat to the fuel cell during single pass mode.

FIG. 2 shows a schematic example of the operation of an MCFC forgeneration of electrical power. In FIG. 2, the anode portion of the fuelcell can receive fuel and steam (H₂O) as inputs, with outputs of water,CO₂, and optionally excess H₂, CH₄ (or other hydrocarbons), and/or CO.The cathode portion of the fuel cell can receive CO₂ and some oxidant(e.g., air/O₂) as inputs, with an output corresponding to a reducedamount of CO₂ in O₂-depleted oxidant (air). Within the fuel cell, CO₃ ²⁻ions formed in the cathode side can be transported across theelectrolyte to provide the carbonate ions needed for the reactionsoccurring at the anode.

Several reactions can occur within a molten carbonate fuel cell such asthe example fuel cell shown in FIG. 2. The reforming reactions can beoptional, and can be reduced or eliminated if sufficient H₂ is provideddirectly to the anode. The following reactions are based on CH₄, butsimilar reactions can occur when other fuels are used in the fuel cell.

(1) <anode reforming>CH₄+H₂O=>3H₂+CO

(2) <water gas shift>CO+H₂O=>H₂+CO₂

(3) <reforming and water gas shift combined>CH₄+2H₂O=>4H₂+CO₂

(4) <anode H₂ oxidation>H₂+CO₃ ²⁻=>H₂O+CO₂+2e⁻

(5) <cathode>½O₂+CO₂+2e⁻=>CO₃ ²⁻

Reaction (1) represents the basic hydrocarbon reforming reaction togenerate H₂ for use in the anode of the fuel cell. The CO formed inreaction (1) can be converted to H₂ by the water-gas shift reaction (2).The combination of reactions (1) and (2) is shown as reaction (3).Reactions (1) and (2) can occur external to the fuel cell, and/or thereforming can be performed internal to the anode.

Reactions (4) and (5), at the anode and cathode respectively, representthe reactions that can result in electrical power generation within thefuel cell. Reaction (4) combines H₂, either present in the feed oroptionally generated by reactions (1) and/or (2), with carbonate ions toform H₂O, CO₂, and electrons to the circuit. Reaction (5) combines O₂,CO₂, and electrons from the circuit to form carbonate ions. Thecarbonate ions generated by reaction (5) can be transported across theelectrolyte of the fuel cell to provide the carbonate ions needed forreaction (4). In combination with the transport of carbonate ions acrossthe electrolyte, a closed current loop can then be formed by providingan electrical connection between the anode and cathode.

In various embodiments, a goal of operating the fuel cell can be toimprove the total efficiency of the fuel cell and/or the totalefficiency of the fuel cell plus an integrated chemical synthesisprocess. This is typically in contrast to conventional operation of afuel cell, where the goal can be to operate the fuel cell with highelectrical efficiency for using the fuel provided to the cell forgeneration of electrical power. As defined above, total fuel cellefficiency may be determined by dividing the electric output of the fuelcell plus the lower heating value of the fuel cell outputs by the lowerheating value of the input components for the fuel cell. In other words,TFCE=(LHV(el)+LHV(sg out))/LHV(in), where LHV(in) and LHV(sg out) referto the LHV of the fuel components (such as H₂, CH₄, and/or CO) deliveredto the fuel cell and syngas (H₂, CO and/or CO₂) in the anode outletstreams or flows, respectively. This can provide a measure of theelectric energy plus chemical energy generated by the fuel cell and/orthe integrated chemical process. It is noted that under this definitionof total efficiency, heat energy used within the fuel cell and/or usedwithin the integrated fuel cell/chemical synthesis system can contributeto total efficiency. However, any excess heat exchanged or otherwisewithdrawn from the fuel cell or integrated fuel cell/chemical synthesissystem is excluded from the definition. Thus, if excess heat from thefuel cell is used, for example, to generate steam for electricitygeneration by a steam turbine, such excess heat is excluded from thedefinition of total efficiency.

Several operational parameters may be manipulated to operate a fuel cellwith excess reformable fuel. Some parameters can be similar to thosecurrently recommended for fuel cell operation. In some aspects, thecathode conditions and temperature inputs to the fuel cell can besimilar to those recommended in the literature. For example, the desiredelectrical efficiency and the desired total fuel cell efficiency may beachieved at a range of fuel cell operating temperatures typical formolten carbonate fuel cells. In typical operation, the temperature canincrease across the fuel cell.

In other aspects, the operational parameters of the fuel cell candeviate from typical conditions so that the fuel cell is operated toallow a temperature decrease from the anode inlet to the anode outletand/or from the cathode inlet to the cathode outlet. For example, thereforming reaction to convert a hydrocarbon into H₂ and CO is anendothermic reaction. If a sufficient amount of reforming is performedin a fuel cell anode relative to the amount of oxidation of hydrogen togenerate electrical current, the net heat balance in the fuel cell canbe endothermic. This can cause a temperature drop between the inlets andoutlets of a fuel cell. During endothermic operation, the temperaturedrop in the fuel cell can be controlled so that the electrolyte in thefuel cell remains in a molten state.

Parameters that can be manipulated in a way so as to differ from thosecurrently recommended can include the amount of fuel provided to theanode, the composition of the fuel provided to the anode, and/or theseparation and capture of syngas in the anode output without significantrecycling of syngas from the anode exhaust to either the anode input orthe cathode input. In some aspects, no recycle of syngas or hydrogenfrom the anode exhaust to either the anode input or the cathode inputcan be allowed to occur, either directly or indirectly. In additional oralternative aspects, a limited amount of recycle can occur. In suchaspects, the amount of recycle from the anode exhaust to the anode inputand/or the cathode input can be less than about 10 vol % of the anodeexhaust, such as less than about 5 vol %, or less than about 1 vol %.

Additionally or alternately, a goal of operating a fuel cell can be toseparate CO₂ from the output stream of a combustion reaction or anotherprocess that produces a CO₂ output stream, in addition to allowinggeneration of electric power. In such aspects, the combustionreaction(s) can be used to power one or more generators or turbines,which can provide a majority of the power generated by the combinedgenerator/fuel cell system. Rather than operating the fuel cell tooptimize power generation by the fuel cell, the system can instead beoperated to improve the capture of carbon dioxide from thecombustion-powered generator while reducing or minimizing the number offuels cells required for capturing the carbon dioxide. Selecting anappropriate configuration for the input and output flows of the fuelcell, as well as selecting appropriate operating conditions for the fuelcell, can allow for a desirable combination of total efficiency andcarbon capture.

In some embodiments, the fuel cells in a fuel cell array can be arrangedso that only a single stage of fuel cells (such as fuel cell stacks) canbe present. In this type of embodiment, the anode fuel utilization forthe single stage can represent the anode fuel utilization for the array.Another option can be that a fuel cell array can contain multiple stagesof anodes and multiple stages of cathodes, with each anode stage havinga fuel utilization within the same range, such as each anode stagehaving a fuel utilization within 10% of a specified value, for examplewithin 5% of a specified value. Still another option can be that eachanode stage can have a fuel utilization equal to a specified value orlower than the specified value by less than an amount, such as havingeach anode stage be not greater than a specified value by 10% or less,for example, by 5% or less. As an illustrative example, a fuel cellarray with a plurality of anode stages can have each anode stage bewithin about 10% of 50% fuel utilization, which would correspond to eachanode stage having a fuel utilization between about 40% and about 60%.As another example, a fuel cell array with a plurality of stages canhave each anode stage be not greater than 60% anode fuel utilizationwith the maximum deviation being about 5% less, which would correspondto each anode stage having a fuel utilization between about 55% to about60%. In still another example, one or more stages of fuel cells in afuel cell array can be operated at a fuel utilization from about 30% toabout 50%, such as operating a plurality of fuel cell stages in thearray at a fuel utilization from about 30% to about 50%. More generally,any of the above types of ranges can be paired with any of the anodefuel utilization values specified herein.

Still another additional or alternate option can include specifying afuel utilization for less than all of the anode stages. For example, insome aspects of the invention fuel cells/stacks can be arranged at leastpartially in one or more series arrangements such that anode fuelutilization can be specified for the first anode stage in a series, thesecond anode stage in a series, the final anode stage in a series, orany other convenient anode stage in a series. As used herein, the“first” stage in a series corresponds to the stage (or set of stages, ifthe arrangement contains parallel stages as well) to which input isdirectly fed from the fuel source(s), with later (“second,” “third,”“final,” etc.) stages representing the stages to which the output fromone or more previous stages is fed, instead of directly from therespective fuel source(s). In situations where both output from previousstages and input directly from the fuel source(s) are co-fed into astage, there can be a “first” (set of) stage(s) and a “last” (set of)stage(s), but other stages (“second,” “third,” etc.) can be more trickyamong which to establish an order (e.g., in such cases, ordinal ordercan be determined by concentration levels of one or more components inthe composite input feed composition, such as CO₂ for instance, fromhighest concentration “first” to lowest concentration “last” withapproximately similar compositional distinctions representing the sameordinal level.)

Yet another additional or alternate option can be to specify the anodefuel utilization corresponding to a particular cathode stage (again,where fuel cells/stacks can be arranged at least partially in one ormore series arrangements). As noted above, based on the direction of theflows within the anodes and cathodes, the first cathode stage may notcorrespond to (be across the same fuel cell membrane from) the firstanode stage. Thus, in some aspects of the invention, the anode fuelutilization can be specified for the first cathode stage in a series,the second cathode stage in a series, the final cathode stage in aseries, or any other convenient cathode stage in a series.

Yet still another additional or alternate option can be to specify anoverall average of fuel utilization over all fuel cells in a fuel cellarray. In various aspects, the overall average of fuel utilization for afuel cell array can be about 65% or less, for example, about 60% orless, about 55% or less, about 50% or less, or about 45% or less(additionally or alternately, the overall average fuel utilization for afuel cell array can be at least about 25%, for example at least about30%, at least about 35%, or at least about 40%). Such an average fuelutilization need not necessarily constrain the fuel utilization in anysingle stage, so long as the array of fuel cells meets the desired fuelutilization.

Applications for CO₂ Output after Capture

In various aspects of the invention, the systems and methods describedabove can allow for production of carbon dioxide as a pressurized fluid.For example, the CO₂ generated from a cryogenic separation stage caninitially correspond to a pressurized CO₂ liquid with a purity of atleast about 90%, e.g., at least about 95%, at least about 97%, at leastabout 98%, or at least about 99%. This pressurized CO₂ stream can beused, e.g., for injection into wells in order to further enhance oil orgas recovery such as in secondary oil recovery. When done in proximityto a facility that encompasses a gas turbine, the overall system maybenefit from additional synergies in use of electrical/mechanical powerand/or through heat integration with the overall system.

Alternatively, for systems dedicated to an enhanced oil recovery (EOR)application (i.e., not comingled in a pipeline system with tightcompositional standards), the CO₂ separation requirements may besubstantially relaxed. The EOR application can be sensitive to thepresence of O₂, so O₂ can be absent, in some embodiments, from a CO₂stream intended for use in EOR. However, the EOR application can tend tohave a low sensitivity to dissolved CO, H₂, and/or CH₄. Also, pipelinesthat transport the CO₂ can be sensitive to these impurities. Thosedissolved gases can typically have only subtle impacts on thesolubilizing ability of CO₂ used for EOR. Injecting gases such as CO,H₂, and/or CH₄ as EOR gases can result in some loss of fuel valuerecovery, but such gases can be otherwise compatible with EORapplications.

Additionally or alternately, a potential use for CO₂ as a pressurizedliquid can be as a nutrient in biological processes such as algaegrowth/harvesting. The use of MCFCs for CO₂ separation can ensure thatmost biologically significant pollutants could be reduced to acceptablylow levels, resulting in a CO₂-containing stream having only minoramounts of other “contaminant” gases (such as CO, H₂, N₂, and the like,and combinations thereof) that are unlikely to substantially negativelyaffect the growth of photosynthetic organisms. This can be in starkcontrast to the output streams generated by most industrial sources,which can often contain potentially highly toxic material such as heavymetals.

In this type of aspect of the invention, the CO₂ stream generated byseparation of CO₂ in the anode loop can be used to produce biofuelsand/or chemicals, as well as precursors thereof. Further additionally oralternately, CO₂ may be produced as a dense fluid, allowing for mucheasier pumping and transport across distances, e.g., to large fields ofphotosynthetic organisms. Conventional emission sources can emit hot gascontaining modest amounts of CO₂ (e.g., about 4-15%) mixed with othergases and pollutants. These materials would normally need to be pumpedas a dilute gas to an algae pond or biofuel “farm”. By contrast, theMCFC system according to the invention can produce a concentrated CO₂stream (−60-70% by volume on a dry basis) that can be concentratedfurther to 95%+(for example 96%+, 97%+, 98%+, or 99%+) and easilyliquefied. This stream can then be transported easily and efficientlyover long distances at relatively low cost and effectively distributedover a wide area. In these embodiments, residual heat from thecombustion source/MCFC may be integrated into the overall system aswell.

An alternative embodiment may apply where the CO₂ source/MCFC andbiological/chemical production sites are co-located. In that case, onlyminimal compression may be necessary (i.e., to provide enough CO₂pressure to use in the biological production, e.g., from about 15 psigto about 150 psig). Several novel arrangements can be possible in such acase. Secondary reforming may optionally be applied to the anode exhaustto reduce CH₄ content, and water-gas shift may optionally additionallyor alternately be present to drive any remaining CO into CO₂ and H₂.

The components from an anode output stream and/or cathode output streamcan be used for a variety of purposes. One option can be to use theanode output as a source of hydrogen, as described above. For an MCFCintegrated with or co-located with a refinery, the hydrogen can be usedas a hydrogen source for various refinery processes, such ashydroprocessing. Another option can be to additionally or alternatelyuse hydrogen as a fuel source where the CO₂ from combustion has alreadybeen “captured.” Such hydrogen can be used in a refinery or otherindustrial setting as a fuel for a boiler, furnace, and/or fired heater,and/or the hydrogen can be used as a feed for an electric powergenerator, such as a turbine. Hydrogen from an MCFC fuel cell canfurther additionally or alternately be used as an input stream for othertypes of fuel cells that require hydrogen as an input, possiblyincluding vehicles powered by fuel cells. Still another option can be toadditionally or alternately use syngas generated as an output from anMCFC fuel cell as a fermentation input.

Another option can be to additionally or alternately use syngasgenerated from the anode output. Of course, syngas can be used as afuel, although a syngas based fuel can still lead to some CO₂ productionwhen burned as fuel. In other aspects, a syngas output stream can beused as an input for a chemical synthesis process. One option can be toadditionally or alternately use syngas for a Fischer-Tropsch typeprocess, and/or another process where larger hydrocarbon molecules areformed from the syngas input. Another option can be to additionally oralternately use syngas to form an intermediate product such as methanol.Methanol could be used as the final product, but in other aspectsmethanol generated from syngas can be used to generate larger compounds,such as gasoline, olefins, aromatics, and/or other products. It is notedthat a small amount of CO₂ can be acceptable in the syngas feed to amethanol synthesis process, and/or to a Fischer-Tropsch processutilizing a shifting catalyst. Hydroformylation is an additional oralternate example of still another synthesis process that can make useof a syngas input.

It is noted that one variation on use of an MCFC to generate syngas canbe to use MCFC fuel cells as part of a system for processing methaneand/or natural gas withdrawn by an offshore oil platform or otherproduction system that is a considerable distance from its ultimatemarket. Instead of attempting to transport the gas phase output from awell, or attempting to store the gas phase product for an extendedperiod, the gas phase output from a well can be used as the input to anMCFC fuel cell array. This can lead to a variety of benefits. First, theelectric power generated by the fuel cell array can be used as a powersource for the platform. Additionally, the syngas output from the fuelcell array can be used as an input for a Fischer-Tropsch process at theproduction site. This can allow for formation of liquid hydrocarbonproducts more easily transported by pipeline, ship, or railcar from theproduction site to, for example, an on-shore facility or a largerterminal

Still other integration options can additionally or alternately includeusing the cathode output as a source of higher purity, heated nitrogen.The cathode input can often include a large portion of air, which meansa substantial portion of nitrogen can be included in the cathode input.The fuel cell can transport CO₂ and O₂ from the cathode across theelectrolyte to the anode, and the cathode outlet can have lowerconcentrations of CO₂ and O₂, and thus a higher concentration of N₂ thanfound in air. With subsequent removal of the residual O₂ and CO₂, thisnitrogen output can be used as an input for production of ammonia orother nitrogen-containing chemicals, such as urea, ammonium nitrate,and/or nitric acid. It is noted that urea synthesis could additionallyor alternately use CO₂ separate from the anode output as an input feed.

Integration Example: Applications for Integration with CombustionTurbines

In some aspects of the invention, a combustion source for generatingpower and exhausting a CO₂-containing exhaust can be integrated with theoperation of molten carbonate fuel cells. An example of a suitablecombustion source is a gas turbine. Preferably, the gas turbine cancombust natural gas, methane gas, or another hydrocarbon gas in acombined cycle mode integrated with steam generation and heat recoveryfor additional efficiency. Modern natural gas combined cycleefficiencies are about 60% for the largest and newest designs. Theresulting CO₂-containing exhaust gas stream can be produced at anelevated temperature compatible with the MCFC operation, such as 300°C.-700° C. and preferably 500° C.-650° C. The gas source can optionallybut preferably be cleaned of contaminants such as sulfur that can poisonthe MCFC before entering the turbine. Alternatively, the gas source canbe a coal-fired generator, wherein the exhaust gas would typically becleaned post-combustion due to the greater level of contaminants in theexhaust gas. In such an alternative, some heat exchange to/from the gasmay be necessary to enable clean-up at lower temperatures. In additionalor alternate embodiments, the source of the CO₂-containing exhaust gascan be the output from a boiler, combustor, or other heat source thatburns carbon-rich fuels. In other additional or alternate embodiments,the source of the CO₂-containing exhaust gas can be bio-produced CO₂ incombination with other sources.

For integration with a combustion source, some alternativeconfigurations for processing of a fuel cell anode can be desirable. Forexample, an alternative configuration can be to recycle at least aportion of the exhaust from a fuel cell anode to the input of a fuelcell anode. The output stream from an MCFC anode can include H₂O, CO₂,optionally CO, and optionally but typically unreacted fuel (such as H₂or CH₄) as the primary output components. Instead of using this outputstream as an external fuel stream and/or an input stream for integrationwith another process, one or more separations can be performed on theanode output stream in order to separate the CO₂ from the componentswith potential fuel value, such as H₂ or CO. The components with fuelvalue can then be recycled to the input of an anode.

This type of configuration can provide one or more benefits. First, CO₂can be separated from the anode output, such as by using a cryogenic CO₂separator. Several of the components of the anode output (H₂, CO, CH₄)are not easily condensable components, while CO₂ and H₂O can beseparated individually as condensed phases. Depending on the embodiment,at least about 90 vol % of the CO₂ in the anode output can be separatedto form a relatively high purity CO₂ output stream. Alternatively, insome aspects less CO₂ can be removed from the anode output, so thatabout 50 vol % to about 90 vol % of the CO₂ in the anode output can beseparated out, such as about 80 vol % or less or about 70 vol % or less.After separation, the remaining portion of the anode output cancorrespond primarily to components with fuel value, as well as reducedamounts of CO₂ and/or H₂O. This portion of the anode output afterseparation can be recycled for use as part of the anode input, alongwith additional fuel. In this type of configuration, even though thefuel utilization in a single pass through the MCFC(s) may be low, theunused fuel can be advantageously recycled for another pass through theanode. As a result, the single-pass fuel utilization can be at a reducedlevel, while avoiding loss (exhaust) of unburned fuel to theenvironment.

Additionally or alternatively to recycling a portion of the anodeexhaust to the anode input, another configuration option can be to use aportion of the anode exhaust as an input for a combustion reaction for aturbine or other combustion device, such as a boiler, furnace, and/orfired heater. The relative amounts of anode exhaust recycled to theanode input and/or as an input to the combustion device can be anyconvenient or desirable amount. If the anode exhaust is recycled to onlyone of the anode input and the combustion device, the amount of recyclecan be any convenient amount, such as up to 100% of the portion of theanode exhaust remaining after any separation to remove CO₂ and/or H₂O.When a portion of the anode exhaust is recycled to both the anode inputand the combustion device, the total recycled amount by definition canbe 100% or less of the remaining portion of anode exhaust. Otherwise,any convenient split of the anode exhaust can be used. In variousembodiments of the invention, the amount of recycle to the anode inputcan be at least about 10% of the anode exhaust remaining afterseparations, for example at least about 25%, at least about 40%, atleast about 50%, at least about 60%, at least about 75%, or at leastabout 90%. Additionally or alternately in those embodiments, the amountof recycle to the anode input can be about 90% or less of the anodeexhaust remaining after separations, for example about 75% or less,about 60% or less, about 50% or less, about 40% or less, about 25% orless, or about 10% or less. Further additionally or alternately, invarious embodiments of the invention, the amount of recycle to thecombustion device can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the combustion device can be about 90% or lessof the anode exhaust remaining after separations, for example about 75%or less, about 60% or less, about 50% or less, about 40% or less, about25% or less, or about 10% or less.

In still other alternative aspects of the invention, the fuel for acombustion device can additionally or alternately be a fuel with anelevated quantity of components that are inert and/or otherwise act as adiluent in the fuel. CO₂ and N₂ are examples of components in a naturalgas feed that can be relatively inert during a combustion reaction. Whenthe amount of inert components in a fuel feed reaches a sufficientlevel, the performance of a turbine or other combustion source can beimpacted. The impact can be due in part to the ability of the inertcomponents to absorb heat, which can tend to quench the combustionreaction. Examples of fuel feeds with a sufficient level of inertcomponents can include fuel feeds containing at least about 20 vol %CO₂, or fuel feeds containing at least about 40 vol % N₂, or fuel feedscontaining combinations of CO₂ and N₂ that have sufficient inert heatcapacity to provide similar quenching ability. (It is noted that CO₂ hasa greater heat capacity than N₂, and therefore lower concentrations ofCO₂ can have a similar impact as higher concentrations of N₂. CO₂ canalso participate in the combustion reactions more readily than N₂, andin doing so remove H₂ from the combustion. This consumption of H₂ canhave a large impact on the combustion of the fuel, by reducing the flamespeed and narrowing the flammability range of the air and fuel mixture.)More generally, for a fuel feed containing inert components that impactthe flammability of the fuel feed, the inert components in the fuel feedcan be at least about 20 vol %, such as at least about 40 vol %, or atleast about 50 vol %, or at least about 60 vol %. Preferably, the amountof inert components in the fuel feed can be about 80 vol % or less.

When a sufficient amount of inert components are present in a fuel feed,the resulting fuel feed can be outside of the flammability window forthe fuel components of the feed. In this type of situation, addition ofH₂ from a recycled portion of the anode exhaust to the combustion zonefor the generator can expand the flammability window for the combinationof fuel feed and H₂, which can allow, for example, a fuel feedcontaining at least about 20 vol % CO₂ or at least about 40% N₂ (orother combinations of CO₂ and N₂) to be successfully combusted.

Relative to a total volume of fuel feed and H₂ delivered to a combustionzone, the amount of H₂ for expanding the flammability window can be atleast about 5 vol % of the total volume of fuel feed plus H₂, such as atleast about 10 vol %, and/or about 25 vol % or less. Another option forcharacterizing the amount of H₂ to add to expand the flammability windowcan be based on the amount of fuel components present in the fuel feedbefore H₂ addition. Fuel components can correspond to methane, naturalgas, other hydrocarbons, and/or other components conventionally viewedas fuel for a combustion-powered turbine or other generator. The amountof H₂ added to the fuel feed can correspond to at least about one thirdof the volume of fuel components (1:3 ratio of H₂:fuel component) in thefuel feed, such as at least about half of the volume of the fuelcomponents (1:2 ratio). Additionally or alternately, the amount of H₂added to the fuel feed can be roughly equal to the volume of fuelcomponents in the fuel feed (1:1 ratio) or less. For example, for a feedcontaining about 30 vol % CH₄, about 10% N₂, and about 60% CO₂, asufficient amount of anode exhaust can be added to the fuel feed toachieve about a 1:2 ratio of H₂ to CH₄. For an idealized anode exhaustthat contained only H₂, addition of H₂ to achieve a 1:2 ratio wouldresult in a feed containing about 26 vol % CH₄, 13 vol % H₂, 9 vol % N₂,and 52 vol % CO₂.

Exhaust Gas Recycle

Aside from providing exhaust gas to a fuel cell array for capture andeventual separation of the CO₂, an additional or alternate potential usefor exhaust gas can include recycle back to the combustion reaction toincrease the CO₂ content. When hydrogen is available for addition to thecombustion reaction, such as hydrogen from the anode exhaust of the fuelcell array, further benefits can be gained from using recycled exhaustgas to increase the CO₂ content within the combustion reaction.

In various aspects of the invention, the exhaust gas recycle loop of apower generation system can receive a first portion of the exhaust gasfrom combustion, while the fuel cell array can receive a second portion.The amount of exhaust gas from combustion recycled to the combustionzone of the power generation system can be any convenient amount, suchas at least about 15% (by volume), for example at least about 25%, atleast about 35%, at least about 45%, or at least about 50%. Additionallyor alternately, the amount of combustion exhaust gas recirculated to thecombustion zone can be about 65% (by volume) or less, e.g., about 60% orless, about 55% or less, about 50% or less, or about 45% or less.

In one or more aspects of the invention, a mixture of an oxidant (suchas air and/or oxygen-enriched air) and fuel can be combusted and(simultaneously) mixed with a stream of recycled exhaust gas. The streamof recycled exhaust gas, which can generally include products ofcombustion such as CO₂, can be used as a diluent to control, adjust, orotherwise moderate the temperature of combustion and of the exhaust thatcan enter the succeeding expander. As a result of using oxygen-enrichedair, the recycled exhaust gas can have an increased CO₂ content, therebyallowing the expander to operate at even higher expansion ratios for thesame inlet and discharge temperatures, thereby enabling significantlyincreased power production.

A gas turbine system can represent one example of a power generationsystem where recycled exhaust gas can be used to enhance the performanceof the system. The gas turbine system can have a first/main compressorcoupled to an expander via a shaft. The shaft can be any mechanical,electrical, or other power coupling, thereby allowing a portion of themechanical energy generated by the expander to drive the maincompressor. The gas turbine system can also include a combustion chamberconfigured to combust a mixture of a fuel and an oxidant. In variousaspects of the invention, the fuel can include any suitable hydrocarbongas/liquid, such as syngas, natural gas, methane, ethane, propane,butane, naphtha diesel, kerosene, aviation fuel, coal derived fuel,bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof.The oxidant can, in some embodiments, be derived from a second or inletcompressor fluidly coupled to the combustion chamber and adapted tocompress a feed oxidant. In one or more embodiments of the invention,the feed oxidant can include atmospheric air and/or enriched air. Whenthe oxidant includes enriched air alone or a mixture of atmospheric airand enriched air, the enriched air can be compressed by the inletcompressor (in the mixture, either before or after being mixed with theatmospheric air). The enriched air and/or the air-enriched air mixturecan have an overall oxygen concentration of at least about 25 volume %,e.g., at least about 30 volume %, at least about 35 volume %, at leastabout 40 volume %, at least about 45 volume %, or at least about 50volume %. Additionally or alternately, the enriched air and/or theair-enriched air mixture can have an overall oxygen concentration ofabout 80 volume % or less, such as about 70 volume % or less.

The enriched air can be derived from any one or more of several sources.For example, the enriched air can be derived from such separationtechnologies as membrane separation, pressure swing adsorption,temperature swing adsorption, nitrogen plant-byproduct streams, and/orcombinations thereof. The enriched air can additionally or alternatelybe derived from an air separation unit (ASU), such as a cryogenic ASU,for producing nitrogen for pressure maintenance or other purposes. Incertain embodiments of the invention, the reject stream from such an ASUcan be rich in oxygen, having an overall oxygen content from about 50volume % to about 70 volume %, can be used as at least a portion of theenriched air and subsequently diluted, if needed, with unprocessedatmospheric air to obtain the desired oxygen concentration.

In addition to the fuel and oxidant, the combustion chamber canoptionally also receive a compressed recycle exhaust gas, such as anexhaust gas recirculation primarily having CO₂ and nitrogen components.The compressed recycle exhaust gas can be derived from the maincompressor, for instance, and adapted to help facilitate combustion ofthe oxidant and fuel, e.g., by moderating the temperature of thecombustion products. As can be appreciated, recirculating the exhaustgas can serve to increase CO₂ concentration.

An exhaust gas directed to the inlet of the expander can be generated asa product of combustion reaction. The exhaust gas can have a heightenedCO₂ content based, at least in part, on the introduction of recycledexhaust gas into the combustion reaction. As the exhaust gas expandsthrough the expander, it can generate mechanical power to drive the maincompressor, to drive an electrical generator, and/or to power otherfacilities.

The power generation system can, in many embodiments, also include anexhaust gas recirculation (EGR) system. In one or more aspects of theinvention, the EGR system can include a heat recovery steam generator(HRSG) and/or another similar device fluidly coupled to a steam gasturbine. In at least one embodiment, the combination of the HRSG and thesteam gas turbine can be characterized as a power-producing closedRankine cycle. In combination with the gas turbine system, the HRSG andthe steam gas turbine can form part of a combined-cycle power generatingplant, such as a natural gas combined-cycle (NGCC) plant. The gaseousexhaust can be introduced to the HRSG in order to generate steam and acooled exhaust gas. The HRSG can include various units for separatingand/or condensing water out of the exhaust stream, transferring heat toform steam, and/or modifying the pressure of streams to a desired level.In certain embodiments, the steam can be sent to the steam gas turbineto generate additional electrical power.

After passing through the HRSG and optional removal of at least someH₂O, the CO₂-containing exhaust stream can, in some embodiments, berecycled for use as an input to the combustion reaction. As noted above,the exhaust stream can be compressed (or decompressed) to match thedesired reaction pressure within the vessel for the combustion reaction.

Example of Integrated System

FIG. 8 schematically shows an example of an integrated system includingintroduction of both CO₂-containing recycled exhaust gas and H₂ or COfrom the fuel cell anode exhaust into the combustion reaction forpowering a turbine. In FIG. 8, the turbine can include a compressor 802,a shaft 804, an expander 806, and a combustion zone 815. An oxygensource 811 (such as air and/or oxygen-enriched air) can be combined withrecycled exhaust gas 898 and compressed in compressor 802 prior toentering combustion zone 815. A fuel 812, such as CH₄, and optionally astream containing H₂ or CO 187 can be delivered to the combustion zone.The fuel and oxidant can be reacted in zone 815 and optionally butpreferably passed through expander 806 to generate electric power. Theexhaust gas from expander 806 can be used to form two streams, e.g., aCO₂-containing stream 822 (that can be used as an input feed for fuelcell array 825) and another CO₂-containing stream 892 (that can be usedas the input for a heat recovery and steam generator system 890, whichcan, for example, enable additional electricity to be generated usingsteam turbines 894). After passing through heat recovery system 890,including optional removal of a portion of H₂O from the CO₂-containingstream, the output stream 898 can be recycled for compression incompressor 802 or a second compressor that is not shown. The proportionof the exhaust from expander 806 used for CO₂-containing stream 892 canbe determined based on the desired amount of CO₂ for addition tocombustion zone 815.

As used herein, the EGR ratio is the flow rate for the fuel cell boundportion of the exhaust gas divided by the combined flow rate for thefuel cell bound portion and the recovery bound portion, which is sent tothe heat recovery generator. For example, the EGR ratio for flows shownin FIG. 8 is the flow rate of stream 822 divided by the combined flowrate of streams 822 and 892.

The CO₂-containing stream 822 can be passed into a cathode portion (notshown) of a molten carbonate fuel cell array 825. Based on the reactionswithin fuel cell array 825, CO₂ can be separated from stream 822 andtransported to the anode portion (not shown) of the fuel cell array 825.This can result in a cathode output stream 824 depleted in CO₂. Thecathode output stream 824 can then be passed into a heat recovery (andoptional steam generator) system 850 for generation of heat exchangeand/or additional generation of electricity using steam turbines 854(which may optionally be the same as the aforementioned steam turbines894). After passing through heat recovery and steam generator system850, the resulting flue gas stream 856 can be exhausted to theenvironment and/or passed through another type of carbon capturetechnology, such as an amine scrubber.

After transport of CO₂ from the cathode side to the anode side of fuelcell array 825, the anode output 835 can optionally be passed into awater gas shift reactor 870. Water gas shift reactor 870 can be used togenerate additional H₂ and CO₂ at the expense of CO (and H₂O) present inthe anode output 835. The output from the optional water gas shiftreactor 870 can then be passed into one or more separation stages 840,such as a cold box or a cryogenic separator. This can allow forseparation of an H₂O stream 847 and CO₂ stream 849 from the remainingportion of the anode output. The remaining portion of the anode output885 can include unreacted H₂ generated by reforming but not consumed infuel cell array 825. A first portion 845 of the H₂-containing stream 885can be recycled to the input for the anode(s) in fuel cell array 825. Asecond portion 887 of stream 885 can be used as an input for combustionzone 815. A third portion 865 can be used as is for another purposeand/or treated for subsequent further use. Although FIG. 8 and thedescription herein schematically details up to three portions, it iscontemplated that only one of these three portions can be exploited,only two can be exploited, or all three can be exploited according tothe invention.

In FIG. 8, the exhaust for the exhaust gas recycle loop is provided by afirst heat recovery and steam generator system 890, while a second heatrecovery and steam generator system 850 can be used to capture excessheat from the cathode output of the fuel cell array 825.

FIG. 9 shows an alternative embodiment where the exhaust gas recycleloop is provided by the same heat recovery steam generator used forprocessing the fuel cell array output. In FIG. 9, recycled exhaust gas998 is provided by heat recovery and steam generator system 950 as aportion of the flue gas stream 956. This can eliminate the separate heatrecovery and steam generator system associated with the turbine.

In various embodiments of the invention, the process can be approachedas starting with a combustion reaction for powering a turbine, aninternal combustion engine, or another system where heat and/or pressuregenerated by a combustion reaction can be converted into another form ofpower. The fuel for the combustion reaction can comprise or be hydrogen,a hydrocarbon, and/or any other compound containing carbon that can beoxidized (combusted) to release energy. Except for when the fuelcontains only hydrogen, the composition of the exhaust gas from thecombustion reaction can have a range of CO₂ contents, depending on thenature of the reaction (e.g., from at least about 2 vol % to about 25vol % or less). Thus, in certain embodiments where the fuel iscarbonaceous, the CO₂ content of the exhaust gas can be at least about 2vol %, for example at least about 4 vol %, at least about 5 vol %, atleast about 6 vol %, at least about 8 vol %, or at least about 10 vol %.Additionally or alternately in such carbonaceous fuel embodiments, theCO₂ content can be about 25 vol % or less, for example about 20 vol % orless, about 15 vol % or less, about 10 vol % or less, about 7 vol % orless, or about 5 vol % or less. Exhaust gases with lower relative CO₂contents (for carbonaceous fuels) can correspond to exhaust gases fromcombustion reactions on fuels such as natural gas with lean (excess air)combustion. Higher relative CO₂ content exhaust gases (for carbonaceousfuels) can correspond to optimized natural gas combustion reactions,such as those with exhaust gas recycle, and/or combustion of fuels suchas coal.

In some aspects of the invention, the fuel for the combustion reactioncan contain at least about 90 volume % of compounds containing fivecarbons or less, e.g., at least about 95 volume %. In such aspects, theCO₂ content of the exhaust gas can be at least about 4 vol %, forexample at least about 5 vol %, at least about 6 vol %, at least about 7vol %, or at least about 7.5 vol %. Additionally or alternately, the CO₂content of the exhaust gas can be about 13 vol % or less, e.g., about 12vol % or less, about 10 vol % or less, about 9 vol % or less, about 8vol % or less, about 7 vol % or less, or about 6 vol % or less. The CO₂content of the exhaust gas can represent a range of values depending onthe configuration of the combustion-powered generator. Recycle of anexhaust gas can be beneficial for achieving a CO₂ content of at leastabout 6 vol %, while addition of hydrogen to the combustion reaction canallow for further increases in CO₂ content to achieve a CO₂ content ofat least about 7.5 vol %.

Alternative Configuration—High Severity NOx Turbine

Gas turbines can be limited in their operation by several factors. Onetypical limitation can be that the maximum temperature in the combustionzone can be controlled below certain limits to achieve sufficiently lowconcentrations of nitrogen oxides (NOx) in order to satisfy regulatoryemission limits. Regulatory emission limits can require a combustionexhaust to have a NOx content of about 20 vppm or less, and possible 10vppm or less, when the combustion exhaust is allowed to exit to theenvironment.

NOx formation in natural gas-fired combustion turbines can be a functionof temperature and residence time. Reactions that result in formation ofNOx can be of reduced and/or minimal importance below a flametemperature of about 1500° F., but NOx production can increase rapidlyas the temperature increases beyond this point. In a gas turbine,initial combustion products can be mixed with extra air to cool themixture to a temperature around 1200° F., and temperature can be limitedby the metallurgy of the expander blades. Early gas turbines typicallyexecuted the combustion in diffusion flames that had stoichiometriczones with temperatures well above 1500° F., resulting in higher NOxconcentrations. More recently, the current generation of ‘Dry Low Nox’(DLN) burners can use special pre-mixed burners to burn natural gas atcooler lean (less fuel than stoichiometric) conditions. For example,more of the dilution air can be mixed in to the initial flame, and lesscan be mixed in later to bring the temperature down to the ˜1200° F.turbine-expander inlet temperature. The disadvantages for DLN burnerscan include poor performance at turndown, higher maintenance, narrowranges of operation, and poor fuel flexibility. The latter can be aconcern, as DLN burners can be more difficult to apply to fuels ofvarying quality (or difficult to apply at all to liquid fuels). For lowBTU fuels, such as fuels containing a high content of CO₂, DLN burnersare typically not used and instead diffusion burners can be used. Inaddition, gas turbine efficiency can be increased by using a higherturbine-expander inlet temperature. However, because there can be alimited amount of dilution air, and this amount can decrease withincreased turbine-expander inlet temperature, the DLN burner can becomeless effective at maintaining low NOx as the efficiency of the gasturbine improves.

In various aspects of the invention, a system integrating a gas turbinewith a fuel cell for carbon capture can allow use of higher combustionzone temperatures while reducing and/or minimizing additional NOxemissions, as well as enabling DLN-like NOx savings via use of turbinefuels that are not presently compatible with DLN burners. In suchaspects, the turbine can be run at higher power (i.e., highertemperature) resulting in higher NOx emissions, but also higher poweroutput and potentially higher efficiency. In some aspects of theinvention, the amount of NOx in the combustion exhaust can be at leastabout 20 vppm, such as at least about 30 vppm, or at least about 40vppm. Additionally or alternately, the amount of NOx in the combustionexhaust can be about 1000 vppm or less, such as about 500 vppm or less,or about 250 vppm or less, or about 150 vppm or less, or about 100 vppmor less. In order to reduce the NOx levels to levels required byregulation, the resulting NOx can be equilibrated via thermal NOxdestruction (reduction of NOx levels to equilibrium levels in theexhaust stream) through one of several mechanisms, such as simplethermal destruction in the gas phase; catalyzed destruction from thenickel cathode catalyst in the fuel cell array; and/or assisted thermaldestruction prior to the fuel cell by injection of small amounts ofammonia, urea, or other reductant. This can be assisted by introductionof hydrogen derived from the anode exhaust. Further reduction of NOx inthe cathode of the fuel cell can be achieved via electrochemicaldestruction wherein the NOx can react at the cathode surface and can bedestroyed. This can result in some nitrogen transport across themembrane electrolyte to the anode, where it may form ammonia or otherreduced nitrogen compounds. With respect to NOx reduction methodsinvolving an MCFC, the expected NOx reduction from a fuel cell/fuel cellarray can be about 80% or less of the NOx in the input to the fuel cellcathode, such as about 70% or less, and/or at least about 5%. It isnoted that sulfidic corrosion can also limit temperatures and affectturbine blade metallurgy in conventional systems. However, the sulfurrestrictions of the MCFC system can typically require reduced fuelsulfur levels that reduce or minimize concerns related to sulfidiccorrosion. Operating the MCFC array at low fuel utilization can furthermitigate such concerns, such as in aspects where a portion of the fuelfor the combustion reaction corresponds to hydrogen from the anodeexhaust.

Operating the Fuel Cell at Low Voltage

The conventional fuel cell practice teaches that molten carbonate andsolid oxide fuel cells should be operated to maximize power density. Theability to maximize power density can be limited by a need to satisfyother operating constraints, such as temperature differential across thefuel cell. Generally, fuel cell parameters are selected to optimizepower density as much as is feasible given other constraints. As anexample, FIG. 6-13 of the NETL Fuel Cell Handbook and the discussionsurrounding FIG. 6-13 teach that operation of a fuel cell at low fuelutilization is hindered by the decrease in fuel conversion that occursas the fuel utilization is decreased. Generally, a higher operatingvoltage V_(A) is desired to increase power density.

An aspect of the invention can be to operate the fuel cell at low fuelutilization, and to overcome the problem of decreased CH₄ conversion bydecreasing the voltage. The decreased voltage can increase the amount ofheat available for use in the conversion reactions. In various aspects,the fuel cell can be operated at a voltage V_(A) of less than about 0.7Volts, for example less than about 0.68 V, less than about 0.67 V, lessthan about 0.66 V, or about 0.65 V or less. Additionally oralternatively, the fuel cell can be operated at a voltage V_(A) of atleast about 0.60, for example at least about 0.61, at least about 0.62,or at least about 0.63. In so doing, energy that would otherwise leavethe fuel cell as electrical energy at high voltage can remain within thecell as heat as the voltage is lowered. This additional heat can allowfor increased endothermic reactions to occur, for example increasing theCH₄ conversion to syngas.

A series of simulations were performed to illustrate the benefits ofoperating a molten carbonate fuel cell according to the invention.Specifically, the simulations were performed to illustrate the benefitof running the fuel cell at lower voltage across different fuelutilizations. The impact of running the fuel cell at lower voltage andlow fuel utilization is shown in FIGS. 16 and 17. FIG. 16 illustrates amodel of the fuel cell in a representation analogous to FIG. 6-13 of theNETL Fuel Cell Handbook. The simulations used to produce the resultsshown in FIG. 16 were run at a constant CH₄ flow rate. FIG. 16 shows theconversion 1620 that can occur at different fuel utilization 1610percentages for different operating voltages. At high voltage (0.8V)1650, as the fuel utilization is decreased, the CH₄ conversion alsoappeared to be decreased to a low level. As the voltage is lowered (to0.7V, 1640, and 0.6V, 1630), the CH₄ conversion at each fuel utilizationpoint modeled appeared to be higher than the corresponding conversion at0.8V. While FIG. 16 shows only a few percentage increase in CH₄conversion, the impact can actually be quite substantial, as illustratedin FIG. 17.

The simulations used to produce the results shown in FIG. 17 were notperformed at a constant flow rate of CH₄, but at a constant fuel cellarea instead. In FIG. 17, the same operation of the fuel cell wasillustrated not on a percentage of CH₄ conversion basis, but on anabsolute flow rate of CH₄ for a fixed fuel cell area. The x-axis 1710shows the fuel utilization and the y-axis 1720 shows normalized CH₄conversion. Plot 1730 shows simulated results produced at 0.6V. Plot1740 shows the simulated results produced at 0.7V. Plot 1750 shows thesimulated results produced at 0.8V. As the fuel utilization isdecreased, and especially as the voltage is decreased, the currentdensity appeared to be increased by more than a factor of 5 for the datashown in FIGS. 16 and 17. As such, the power density can be increased bylowering the operating voltage under operating conditions consistentwith aspects of the invention. The increased power density and lowervoltage seems to be contrary to the affect achieved during conventionaloperations, where lower operating voltage typically results in lowerpower density. As shown in FIG. 17, the impact on total CH₄ conversionappeared significant: much higher conversion of CH₄, measured as anabsolute flow rate, was achieved at lower fuel utilization when thevoltage was decreased.

Additional Embodiments

Embodiment 1. A method for capturing carbon dioxide from a combustionsource, the method comprising: introducing a fuel stream and anO₂-containing stream into a combustion zone; performing a combustionreaction in the combustion zone to generate a combustion exhaust, thecombustion exhaust comprising CO₂; processing a cathode inlet stream,the cathode inlet stream comprising at least a first portion of thecombustion exhaust, with a fuel cell array of one or more moltencarbonate fuel cells to form a cathode exhaust stream from at least onecathode outlet of the fuel cell array, the one or more molten carbonatefuel cells comprising one or more fuel cell anodes and one or more fuelcell cathodes, the one or more molten carbonate fuel cells beingoperatively connected to the combustion zone through at least onecathode inlet; reacting carbonate from the one or more fuel cellcathodes with H₂ within the one or more fuel cell anodes to produceelectricity and an anode exhaust stream from at least one anode outletof the fuel cell array, the anode exhaust steam comprising CO₂ and H₂;separating CO₂ from the anode exhaust stream in one or more separationstages to form a CO₂-depleted anode exhaust stream; passing at least acombustion-recycle portion of the CO₂-depleted anode exhaust stream tothe combustion zone; and recycling at least an anode-recycle portion ofthe CO₂-depleted anode exhaust stream to the one or more fuel cellanodes.

Embodiment 2

The method of Embodiment 1, wherein a fuel utilization in the one ormore fuel cell anodes is about 65% or less (e.g., about 60% or less).

Embodiment 3

The method of Embodiment 2, wherein the fuel utilization in the one ormore fuel cell anodes is about 30% to about 50%.

Embodiment 4

The method of claim Embodiment 2, wherein the one or more fuel cellanodes comprise a plurality of anode stages and the one or more fuelcell cathodes comprise a plurality of cathode stages, wherein a lowutilization anode stage in the plurality of anode stages has an anodefuel utilization of 65% or less (such as about 60% or less), the lowutilization anode stage corresponding to high utilization cathode stageof the plurality of cathode stages, the high utilization cathode stagehaving a CO₂ content at a cathode inlet as high as or higher than a CO₂at a cathode inlet of any other cathode stage of the plurality ofcathode stages.

Embodiment 5

The method of Embodiment 4, wherein the fuel utilization in the lowutilization anode stage is at least about 40%, (e.g., at least about 45%or at least about 50%).

Embodiment 6

The method of Embodiment 4, wherein a fuel utilization in each anodestage of the plurality of anode stages is about 65% or less (e.g., about60% or less).

Embodiment 7

The method of any of the above embodiments, wherein thecombustion-recycle portion of the CO₂-depleted anode exhaust streamcomprises at least about 25% of the CO₂-depleted anode exhaust stream,and wherein the anode-recycle portion of the CO₂-depleted anode exhauststream comprises at least about 25% of the CO₂-depleted anode exhauststream.

Embodiment 8

The method of Embodiment 7, further comprising passing carbon-containingfuel into the one or more fuel cell anodes, the carbon-containing fueloptionally comprising CH₄.

Embodiment 9

The method of Embodiment 8, further comprising: reforming at least aportion of the carbon-containing fuel to generate H₂; and passing atleast a portion of the generated H₂ into the one or more fuel cellanodes.

Embodiment 10

The method of Embodiment 8, wherein the carbon-containing fuel is passedinto the one or more fuel cell anodes without passing thecarbon-containing fuel into a reforming stage prior to entering the oneor more fuel cell anodes.

Embodiment 11

The method of any of the above embodiments, wherein the combustionexhaust comprises about 10 vol % or less of CO₂ (e.g., 8 vol % or lessof CO₂), the combustion exhaust optionally comprising at least about 4vol % of CO₂

Embodiment 12

The method of any of the above Embodiments, further comprising recyclinga second portion of the combustion exhaust to the combustion zone, thesecond portion of the combustion exhaust optionally comprising at leastabout 6 vol % CO₂.

Embodiment 13

The method of Embodiment 12, wherein recycling the second portion of thecombustion exhaust to the combustion zone comprises: exchanging heatbetween a second portion of the combustion exhaust and an H₂O-containingstream to form steam; separating water from the second portion of thecombustion exhaust to form an H₂O-depleted combustion exhaust stream;and passing at least a portion of the H₂O-depleted combustion exhaustinto the combustion zone.

Embodiment 14

The method of any of the above embodiments, wherein the anode exhauststream, prior to the separating CO₂ from the anode exhaust stream in oneor more separation stages, comprises at least about 5.0 vol % of H₂(e.g., at least about 10 vol % or at least about 15 vol %).

Embodiment 15

The method of any of the above embodiments, further comprising exposingthe anode exhaust stream to a water gas shift catalyst to form a shiftedanode exhaust stream prior to the separating CO₂ from the anode exhauststream in one or more separation stages, a H₂ content of the shiftedanode exhaust stream after exposure to the water gas shift catalystbeing greater than a H₂ content of the anode exhaust stream prior toexposure to the water gas shift catalyst.

Embodiment 16

The method of any of the above Embodiments, wherein thecombustion-recycle portion of the CO₂-depleted anode exhaust stream iscombined with the fuel stream prior to passing the combustion-recycleportion of the CO₂-depleted anode exhaust stream to the combustion zone.

Embodiment 17

The method of any of the above embodiments, wherein a cathode exhauststream has a CO₂ content of about 2.0 vol % or less (e.g., about 1.5 vol% or less or about 1.2 vol % or less).

Embodiment 18

The method of any of the above embodiments, wherein separating CO₂ fromthe anode exhaust stream in one or more separation stages comprises:optionally separating water from the anode exhaust stream to form anoptionally H₂O-depleted anode exhaust stream; cooling the optionallyH₂O-depleted anode exhaust stream to form a condensed phase of CO₂.

EXAMPLES

A series of simulations were performed in order to demonstrate thebenefits of using an improved configuration for using a fuel cell forCO₂ separation. The simulations were based on empirical models for thevarious components in the power generation system. The simulations werebased on determining steady state conditions within a system based onmass balance and energy balance considerations.

For the combustion reaction for the turbine, the model included anexpected combustion energy value and expected combustion products foreach fuel component in the feed to the combustion zone (such as C₁-C₄hydrocarbon, H₂, and/or CO). This was used to determine the combustionexhaust composition. An initial reforming zone prior to the anode can beoperated using an “idealized” reforming reaction to convert CH₄ to H₂.The anode reaction was modeled to also operate to perform furtherreforming during anode operation. It is noted that the empirical modelfor the anode did not require an initial H₂ concentration in the anodefor the reforming in the anode to take place. Both the anode and cathodereactions were modeled to convert expected inputs to expected outputs ata utilization rate that was selected as a model input. The model for theinitial reforming zone and the anode/cathode reactions included anexpected amount of heat energy needed to perform the reactions. Themodel also determined the electrical current generated based on theamount of reactants consumed in the fuel cell and the utilization ratesfor the reactants based on the Nernst equation. For species that wereinput to either the combustion zone or the anode/cathode fuel cell thatdid not directly participate in a reaction within the modeled component,the species were passed through the modeled zone as part of the exhaustor output.

In addition to the chemical reactions, the components of the system hadexpected heat input/output values and efficiencies. For example, thecryogenic separator had an energy that was required based on the volumeof CO₂ and H₂O separated out, as well as an energy that was requiredbased on the volume of gas that was compressed and that remained in theanode output flow. Expected energy consumption was also determined for awater gas shift reaction zone, if present, and for compression ofrecycled exhaust gas. An expected efficiency for electric generationbased on steam generated from heat exchange was also used in the model.

The basic configuration used for the simulations included a combustionturbine combine including a compressor, a combustion zone, and anexpander, similar to the arrangement in FIG. 8. In the baseconfiguration, a natural gas fuel input 812 was provided to thecombustion zone 815. The natural gas input included ˜93% CH₄, ˜2% C₂H₆,˜2% CO₂, and ˜3% N₂. The oxidant feed 811 to the compressor 802 had acomposition representative of air, including about 70% N₂ and about 18%O₂. After passing through the expander 806, a portion 892 of thecombustion exhaust gas was passed through a heat recovery steamgeneration system 890 and then recycled to the compressor 802. Theremainder of the combustion exhaust 822 was passed into the fuel cellcathode. After passing through the fuel cell cathode, the cathodeexhaust 824 exited the system. Unless otherwise specified, the portionof the combustion exhaust 892 recycled back to the combustion zone was˜35%. This recycled portion of the combustion exhaust served to increasethe CO₂ content of the output from the combustion zone. Because the fuelcell area was selected to reduce the CO₂ concentration in the cathodeoutput to a fixed value of ˜1.45%, recycling the combustion exhaust wasfound to improve the CO₂ capture efficiency.

In the base configuration, the fuel cell was modeled as a single fuelcell of an appropriate size to process the combustion exhaust. This wasdone to represent use of a corresponding plurality of fuel cells (fuelcell stacks) arranged in parallel having the same active area as themodeled cell. Unless otherwise specified, the fuel utilization in theanode of the fuel cell was set to ˜75%. The fuel cell area was allowedto vary, so that the selected fuel utilization results in the fuel celloperating at a constant fuel cell voltage of ˜0.7 volts and a constantCO₂ cathode output/exhaust concentration of ˜1.45 vol %.

In addition to the chemical reactions, the components of the system hadexpected heat input/output values and efficiencies. For example, thecryogenic separator had an energy that was required based on the volumeof CO₂ and H₂O separated out, as well as an energy that was requiredbased on the volume of gas that was compressed and that remained in theanode output flow. Expected energy consumption was also determined for awater gas shift reaction zone, if present, and for compression ofrecycled exhaust gas. An expected efficiency for electric generationbased on steam generated from heat exchange was also used in the model.

The basic configuration used for the simulations included a combustionturbine combine including a compressor, a combustion zone, and anexpander. In the base configuration, a natural gas fuel input wasprovided to the combustion zone. The natural gas input included ˜93%CH₄, ˜2% C₂H₆, ˜2% CO₂, and ˜3% N₂. The oxidant feed to the compressorhad a composition representative of air, including about 70% N₂ andabout 18% O₂. After passing through the expander, a portion of thecombustion exhaust gas was passed through a heat recovery steamgeneration system and then recycled to the compressor. The remainder ofthe combustion exhaust was passed into the fuel cell cathode. Afterpassing through the fuel cell cathode, the cathode exhaust exited thesystem. Unless otherwise specified, the portion of the combustionexhaust recycled back to the combustion zone was ˜35%. This recycledportion of the combustion exhaust served to increase the CO₂ content ofthe output from the combustion zone. Because the fuel cell area wasselected to reduce the CO₂ concentration in the cathode output to afixed value of ˜1.45%, recycling the combustion exhaust was found toimprove the CO₂ capture efficiency.

In the base configuration, the fuel cell was modeled as a single fuelcell of an appropriate size to process the combustion exhaust. This wasdone to represent use of a corresponding plurality of fuel cells (fuelcell stacks) arranged in parallel having the same active area as themodeled cell. Unless otherwise specified, the fuel utilization in theanode of the fuel cell was set to ˜75%. The fuel cell area was allowedto vary, so that the selected fuel utilization results in the fuel celloperating at a constant fuel cell voltage of ˜0.7 volts and a constantCO₂ cathode output/exhaust concentration of ˜1.45 vol %.

In the base configuration, an anode fuel input flow provided the naturalgas composition described above as a feed to the anode. Steam was alsopresent to provide a steam to carbon ratio in the input fuel of ˜2:1.Optionally, the natural gas input can undergo reforming to convert aportion of the CH₄ in the natural gas to H₂ prior to entering the anode.When a prior reforming stage is present, ˜20% of the CH₄ could bereformed to generate H₂ prior to entering the anode. The anode outputwas passed through a cryogenic separator for removal of H₂O and CO₂. Theremaining portion of the anode output after separation was processeddepending on the configuration for each Example.

For a given configuration, a variety of values could be calculated atsteady state. For the fuel cell, the amount of CO₂ in the anode exhaustand the amount of O₂ in the cathode exhaust was determined. The voltagefor the fuel cell was fixed at ˜0.7 V within each calculation. Forconditions that could result in a higher maximum voltage, the voltagewas stepped down in exchange for additional current, in order tofacilitate comparison between simulations. The area of fuel cellrequired to achieve a final cathode exhaust CO₂ concentration of ˜1.45vol % was also determined to allow for determination of a currentdensity per fuel cell area.

Another set of values were related to CO₂ emissions. The percentage ofCO₂ captured by the system was determined based on the total CO₂generated versus the amount of CO₂ (in Mtons/year) captured and removedvia the cryogenic separator. The CO₂ not captured corresponded to CO₂“lost” as part of the cathode exhaust. Based on the amount of CO₂captured, the area of fuel cell required per ton of CO₂ captured couldalso be determined

Other values determined in the simulation included the amount of H₂ inthe anode feed relative to the amount of carbon and the amount of N₂ inthe anode feed. It is noted that the natural gas used for both thecombustion zone and the anode feed included a portion of N₂, as would beexpected for a typical real natural gas feed. As a result, N₂ waspresent in the anode feed. The amount of heat (or equivalently steam)required for heating the anode feed for reforming was also determined Asimilar power penalty was determined based on the power required forcompression and separation in the cryogenic separation stages. Forconfigurations where a portion of the anode exhaust was recycled to thecombustion turbine, the percentage of the turbine fuel corresponding toH₂ was also determined. Based on the operation of the turbine, the fuelcell, and the excess steam generated, as well as any power consumed forheating the reforming zone, compression, and/or separation, a total netpower was determined for the system to allow for a net electricalefficiency to be determined based on the amount of natural gas (or otherfuel) used as an input for the turbine and the anode.

FIGS. 10, 11, and 12 show results from simulations performed based onseveral configuration variants. FIG. 10 shows configurationscorresponding to a base configuration as well as several configurationswhere a portion of the anode output was recycled to the anode input. InFIG. 10, a first configuration (1a) was based on passing the remaininganode output after the carbon dioxide and water separation stage(s) intoa combustor located after the turbine combustion zone. This providedheat for the reforming reaction and also provided additional carbondioxide for the cathode input. Configuration 1a was representative of aconventional system, such as the aforementioned Manzolini reference,with the exception that the Manzolini reference did not describe recycleof exhaust gas. Use of the anode output as a feed for the combustorresulted in a predicted fuel cell area of ˜208 km² in order to reducethe CO₂ content of the cathode output to ˜1.45 vol %. The amount of CO₂lost as part of the cathode exhaust was ˜111 lbs CO₂/MWhr. Due to thelarge fuel cell area required for capturing the CO₂, the net powergenerated was ˜724 MW per hour. Based on these values, the amount offuel cell area needed to capture a fixed amount of CO₂ could becalculated, such as an area of fuel cell needed to capture a megaton ofCO₂ during a year of operation. For Configuration 1a, the area of fuelcell required was ˜101.4 km²*year/Mton-CO₂. The efficiency forgeneration of electrical power relative to the energy content of allfuel used in the power generation system was ˜58.9%. By comparison, theelectrical efficiency for the turbine without any form of carbon capturewas ˜61.1%.

In a second set of configurations (2a-2e), the anode output was recycledto the anode input. Configuration 2a represented a basic recycle of theanode output after separation to the anode input. Configuration 2bincluded a water gas shift reaction zone prior to the carbon dioxideseparation stages. Configuration 2c did not include a reforming stageprior to the anode input. Configuration 2d included a reforming stage,but was operated with a fuel utilization of ˜50% instead of ˜75%.Configuration 2e was operated with a fuel utilization of ˜50% and didnot have a reforming stage prior to the anode.

Recycling the anode output back to the anode input, as shown inConfiguration 2a, resulted in a reduction of the required fuel cell areato ˜161 km². However, the CO₂ loss from the cathode exhaust wasincreased to ˜123 lbs CO₂/MWhr. This was due to the fact that additionalCO₂ was not being added to the cathode input by the combustion of anodeexhaust in a combustor after the turbine. Instead, the CO₂ content ofthe cathode input was based only on the CO₂ output of the combustionzone. The net result in Configuration 2a was a lower area of fuel cellper ton of CO₂ captured of ˜87.5 km²*year/Mton-CO₂, but a modestlyhigher amount of CO₂ emissions. Due to the reduced fuel cell area, thetotal power generated was ˜661 MW. Although the net power generated inConfiguration 2a was about 10% less than the net power in Configuration1a, the fuel cell area was reduced by more than 20%. The electricalefficiency was ˜58.9%.

In Configuration 2b, the additional water gas shift reaction zoneincreased the hydrogen content delivered to the anode, which reduced theamount of fuel needed for the anode reaction. Including the water gasshift reaction zone in Configuration 2b resulted in a reduction of therequired fuel cell area to ˜152 km². The CO₂ loss from the cathodeexhaust was ˜123 lbs CO₂/MWhr. The area of fuel cell per megaton of CO₂captured was ˜82.4 km²*year/Mton-CO₂. The total power generated was ˜664MW. The electrical efficiency was ˜59.1%.

Configuration 2c can take further advantage of the hydrogen content inthe anode recycle by eliminating the reforming of fuel occurring priorto entering the anode. In Configuration 2c, reforming can still occurwithin the anode itself. However, in contrast to a conventional systemincorporating a separate reforming stage prior to entry into the fuelcell anode, Configuration 2c relied on the hydrogen content of therecycled anode gas to provide the minimum hydrogen content forsustaining the anode reaction. Because a separate reforming stage wasnot required, the heat energy was not consumed to maintain thetemperature of the reforming stage. Configuration 2c resulted in areduction of the required fuel cell area to ˜149 km². The CO₂ loss fromthe cathode exhaust was ˜122 lbs CO₂/MWhr. The area of fuel cell per tonof CO₂ captured was ˜80.8 km²*year/Mton-CO₂. The total power generatedwas ˜676 MW.

The electrical efficiency was ˜60.2%. Based on the simulation results,eliminating the reforming step seemed to have only a modest impact onthe required fuel cell area, but the electrical efficiency appeared tobe improved by about 1% relative to Configuration 2b. For an industrialscale power generation plant, an efficiency improvement of even only 1%is believed to represent an enormous advantage over the course of a yearin power generation.

In Configuration 2d, reforming was still performed to convert ˜20% ofthe methane input to the anode into H₂ prior to entering the anode.Instead, the fuel utilization within the anode was reduced from ˜75% to˜50%. This resulted in a substantial reduction of the required fuel cellarea to ˜113 km². The CO₂ loss from the cathode exhaust was ˜123 lbsCO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜61.3km²*year/Mton-CO₂. The total power generated was ˜660 MW. The electricalefficiency was ˜58.8%. Based on the simulation results, reducing thefuel utilization provided a substantial reduction in fuel cell area.Additionally, in comparison with Configurations 2b and 2e, Configuration2d unexpectedly provided the lowest fuel cell area for achieving thedesired level of CO₂ removal.

Configuration 2e incorporated both the reduced fuel utilization of ˜50%as well as elimination of the reforming stage prior to the anode inlet.This configuration provided a combination of improved electricalefficiency and reduced fuel cell area. However, the fuel cell area wasslightly larger than the fuel cell area required in Configuration 2d.This was surprising, as eliminating the reforming stage prior to theanode inlet in Configuration 2c reduced the fuel cell area in comparisonwith Configuration 2b. Based on this, it would have been expected thatConfiguration 2e would provide a further reduction in fuel cell arearelative to Configuration 2d. In Configuration 2e, the CO₂ loss from thecathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell per ton ofCO₂ captured of ˜65.0 km²*year/Mton-CO₂. The total power generated was˜672 MW. The electrical efficiency was ˜59.8%. It is noted thatConfiguration 2d generated only 2% less power than Configuration 2e,while the fuel cell area of Configuration 2d was at least 6% lower thanConfiguration 2e.

The simulation results for Configurations 2b-2e provide a comparison ofhow reducing the anode fuel utilization can impact the total electricalefficiency in a power generation system. Even though reducing the fuelutilization to ˜50% in Configuration 2d led to a reduction in fuel cellarea relative to Configuration 2b, the reduced anode fuel utilizationalso appeared to result in a reduction in electrical efficiency from˜59.1% to ˜58.8%. This was in general agreement with conventional viewson fuel utilization for molten carbonate fuel cells, where high fuelutilization values can be used to allow for efficient use of fueldelivered to the system. In the simulations for Configurations 2b-2e, inorder to achieve an improvement in total electrical efficiency, the lowfuel utilization can be combined with reducing and/or eliminating theamount of reforming, as shown in Configuration 2e.

FIG. 11 shows simulation results for additional configurations thatincluded recycle of at least a portion of the anode exhaust to thecombustion zone for the turbine.

In FIG. 11, Configuration 1b was similar to Configuration 1a (shown inFIG. 10), but also included a water-gas shift reaction stage prior tothe CO₂ separation stages. Thus, Configuration 1b was representative ofa conventional system, such as the aforementioned Manzolini reference,with the exceptions that the Manzolini reference did not describe awater-gas shift reaction stage or recycle of exhaust gas. The requiredfuel cell area to achieve a CO₂ concentration in the cathode exhaust of−1.45% was −190 km². The amount of CO₂ lost as part of the cathodeexhaust was −117 lbs

CO₂/MWhr. The area of fuel cell per ton of CO₂ captured was −97.6km²*year/Mton-CO₂. The total power generated was −702 MW. The electricalefficiency was −59.1%.

Configurations 3a, 3b, and 3d correspond to configurations where theanode output was used as an input for the combustion zone of theturbine. In these configurations, the H₂ content of the anode output wasavailable for use as a fuel in the turbine combustion zone. Thisappeared to be advantageous, as the carbon-containing fuel used togenerate the H₂ was generated in the anode recycle loop, where themajority of the resulting CO₂ can be removed via the cryogenicseparation stages. This could also result in a reduction of the amountof carbon containing fuel delivered to the combustion zone, but thereduction in carbon-containing fuel in the combustion zone could alsoresult in the reduction of the CO₂ concentration in the input to thecathode.

Configuration 3a was a configuration similar to Configuration 1a, butwith recycle of the anode exhaust to the combustion zone. The requiredfuel cell area to achieve a CO₂ concentration in the cathode exhaust of−1.45% was −186 km². The amount of CO₂ lost as part of the cathodeexhaust was −114 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was −100.3 km²*year/Mton-CO₂. The total power generated was−668 MW. The electrical efficiency was −59.7%. Relative to Configuration1a, Configuration 3a had a lower total amount of CO₂ generated (−2.05Mtons/year for Configuration 1a vs.˜1.85 Mtons/year for Configuration3a). This was believed to be due to the reduced amount ofcarbon-containing fuel delivered to the combustion zone. However, thisalso appeared to result in a reduced CO₂ concentration delivered to thecathode input, which caused the model to show a reduced efficiency ofCO₂ removal for Configuration 3a. As a result, the net amount of CO₂exiting in the cathode exhaust was comparable for Configuration 1a andConfiguration 3a. However, Configuration 3a appeared to have severaladvantages relative to Configuration 1a. First, Configuration 3arequired a lower fuel cell area, so that the system in Configuration 3awould likely have a reduced cost. Additionally, the system inConfiguration 3a appeared to have improved electrical efficiency, whichcan indicate lower fuel usage, even after adjusting for the differentpower output of the configurations.

Configuration 3b was similar to Configuration 3a, but also included awater gas shift reaction zone prior to the cryogenic separation stages.The required fuel cell area to achieve a CO₂ concentration in thecathode exhaust of ˜1.45% was ˜173 km². The amount of CO₂ lost as partof the cathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell perton of CO₂ captured was ˜96.1 km²*year/Mton-CO₂. The total powergenerated was ˜658 MW. The electrical efficiency was ˜59.8%.Configuration 3b appeared to have increased CO₂ emission via the cathodeexhaust. This was believed to be due to the additional hydrogendelivered to the combustion zone, which can result in a correspondingreduction in the amount of CO₂ the combustion exhaust used for thecathode input. However, the fuel cell area was further reduced.

Configuration 3d was similar to Configuration 3b, but the anode fuelutilization was reduced from ˜75% to ˜50%. The required fuel cell areato achieve a CO₂ concentration in the cathode exhaust of ˜1.45% was ˜132km². The amount of CO₂ lost as part of the cathode exhaust was ˜128 lbsCO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜77.4km²*year/Mton-CO₂. The total power generated was ˜638 MW. The electricalefficiency was ˜60.7%. Based on the simulation results, reducing thefuel utilization in the anode appeared to result in a substantialimprovement in electrical efficiency relative to Configuration 3b. Thiswas believed to be due to the additional hydrogen delivered to thecombustion zone for the turbine. For comparison, the electricalefficiency of the turbine without any carbon capture was ˜61.1%. Thus,the combination of recycling anode exhaust to the combustion zone andlower fuel utilization appeared to allow an electrical efficiency to beachieved approaching the efficiency without a carbon capture system.

FIG. 12 shows simulation results for additional configurations includingrecycle of at least a portion of the anode exhaust to both thecombustion zone for the turbine and to the anode inlet. Configurations4d, 4e, and 4f represent configurations where the remaining anodeexhaust after separation (removal) of CO₂ and H₂O was divided evenlybetween recycle to the anode input and recycle to the combustion zonefor the turbine. In order to provide sufficient hydrogen for both theanode input and the combustion zone, the anode fuel utilization inConfigurations 4d and 4e was set to ˜50%. Configurations 4d and 4e bothincluded a water gas shift reaction zone prior to the separation stages.Configuration 4d included a separate reforming stage for reforming ˜20%of the additional fuel input to the anode prior to the fuel entering theanode. Configuration 4e did not include a reforming stage prior to thefuel entering the anode input. Configuration 4f was similar toConfiguration 4e, with the exception that the anode fuel utilization inConfiguration 4f was ˜33%, as opposed to the ˜50% in Configuration 4e.

Configuration 4d appeared to show the benefits of recycling the anodeexhaust to both the anode input and the combustion zone. Relative toConfiguration 2d,

Configuration 4d appeared to provide an electrical efficiency about afull percentage point greater. Relative to Configuration 3d,Configuration 4d provided a reduced fuel cell area. In Configuration 4d,the required fuel cell area to achieve a CO₂ concentration in thecathode exhaust of ˜1.45% was ˜122 km². The amount of CO₂ lost as partof the cathode exhaust was ˜126 lbs CO₂/MWhr. The area of fuel cell perton of CO₂ captured was ˜63.4 km²*year/Mton-CO₂. The total powergenerated was ˜650 MW. The electrical efficiency was ˜59.9%.

Removing the pre-reforming stage in Configuration 4e appeared to providefurther benefits. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜112 km². The amountof CO₂ lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. Thearea of fuel cell per ton of CO₂ captured was ˜63.4 km²*year/Mton-CO₂.The total power generated was ˜665 MW. The electrical efficiency was˜61.4%. It is noted that the electrical efficiency was actually greaterthan the efficiency of the turbine without any type of carbon capture(−61.1%).

Reducing the anode fuel utilization in Configuration 4f appeared toprovide still further benefits with regard to both reducing fuel cellarea and increasing electrical efficiency. The required fuel cell areato achieve a CO₂ concentration in the cathode exhaust of ˜1.45% was ˜86km². The amount of CO₂ lost as part of the cathode exhaust was ˜126 lbsCO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜50.6km²*year/Mton-CO₂. The total power generated was ˜654 MW. The electricalefficiency was ˜62.4%. It is noted that the electrical efficiency isactually greater than the efficiency of the turbine without any type ofcarbon capture (61.1%).

Configurations 5d, 5e, and 5f were similar to Configurations 4d, 4e, and4f, with the exception that the exhaust gas recycle rate inConfigurations 5d, 5e, and 5f was increased to ˜45%. Configurations 5d,5e, and 5f had similar fuel cell areas and appeared to provide similarelectrical efficiency, as compared to Configurations 4d, 4e, and 4EHowever, the net amount of CO₂ allowed to leave the system via thecathode exhaust was reduced by about 15% to about 20%, when the exhaustgas recycle rate was increased from about 30% to about 45%.

FIG. 13 shows results from simulations performed based on severalconfiguration variants and alternative operating conditions. Thesimulations of FIG. 13 took into account more factors than thesimulations explained previously with reference FIG. 10. Otherwise, thesimulations shown in FIG. 13 were similar to the simulations shown inFIG. 10, with a few variations added. For example, each case wassimulated at about 0.65 volts in addition to the about 0.7 volts used inthe FIG. 10 simulations. In addition, a case with 0% EGR was added toeach configuration. FIG. 13 shows configurations corresponding to a baseconfiguration as well as several configurations where a portion of theanode output was recycled to the anode input. Unless noted, the exhaustgas recycle was about 35% for the simulated results shown in FIG. 10. InFIG. 13, each configuration was run with either ˜35% or 0% EGR as shown.

In addition to different configurations and alternative operatingconditions, FIG. 13 shows additional parameters that were not shown inFIG. 10. For example, FIG. 13 includes the approximate fuel utilization,approximate steam to carbon ratio, EGR recycle %, whether or not watergas shift reactors were present in the configuration to process theanode exhaust, the approximate internal reforming %, the approximate CO₂concentration in the cathode inlet, and the approximate O₂ content inthe cathode exhaust.

In FIG. 13, a first configuration (O) shown in column 1304 was based onpassing the remaining anode output after the carbon dioxide and waterseparation stage(s) into a combustor located after the turbinecombustion zone. This provided heat for the reforming reaction and alsoprovided additional carbon dioxide for the cathode input. Configuration0 did not include EGR. Configuration 0 provided a useful base case forcomparison with other simulations that did not include EGR.Configuration 0 was representative of a conventional system, such as theaforementioned Manzolini reference. Use of the anode output as a feedfor the combustor resulted in a predicted fuel cell area of −185 km² inorder to reduce the CO₂ content of the cathode output to −1.5 vol %. Theamount of CO₂ lost as part of the cathode exhaust was −212 lbs

CO₂/MWhr. Due to the large fuel cell area required for capturing theCO₂, the net power generated was −679 MW per hour. Based on thesevalues, the amount of fuel cell area needed to capture a fixed amount ofCO₂ could be calculated, such as an area of fuel cell needed to capturea megaton of CO₂ during a year of operation. For Configuration 0, thearea of fuel cell required to capture a megaton was −113.9km²*year/Mton-CO₂. The efficiency for generation of electrical powerrelative to the energy content of all fuel used in the power generationsystem was −57.6%.

In FIG. 13, a second base configuration (1a) shown in column 1306 wasbased on passing the remaining anode output after the carbon dioxide andwater separation stage(s) into a combustor located after the turbinecombustion zone. This provided heat for the reforming reaction and alsoprovided additional carbon dioxide for the cathode input. Configuration1a was representative of a conventional system, such as theaforementioned Manzolini reference, with the exception that theManzolini reference did not describe recycle of exhaust gas. Use of theanode output as a feed for the combustor resulted in a predicted fuelcell area of −215 km² in order to reduce the CO₂ content of the cathodeoutput to −1.5 vol %. The amount of CO₂ lost as part of the cathodeexhaust was −148 lbs CO₂/MWhr. Due to the large fuel cell area requiredfor capturing the CO₂, the net power generated was −611 MW per hour.Based on these values, the amount of fuel cell area needed to capture afixed amount of CO₂ could be calculated, such as an area of fuel cellneeded to capture a megaton of CO₂ during a year of operation. ForConfiguration 1a, the area of fuel cell required to capture a megatonwas −114.2 km²*year/Mton-CO₂. The efficiency for generation ofelectrical power relative to the energy content of all fuel used in thepower generation system was −51.2%. Base case 1a may be compared to basecase 0 to show a result of adding exhaust gas recycle at −35%.

In FIG. 13, a third base configuration (1b) shown in column 1308 wasbased on passing the remaining anode output after the carbon dioxide andwater separation stage(s) into a combustor located after the turbinecombustion zone. This provided heat for the reforming reaction and alsoprovided additional carbon dioxide for the cathode input. Base case 1bincluded water gas shift reactors to process the anode exhaust prior tocarbon dioxide and water separation stage(s). Configuration 1b wasrepresentative of a conventional system, such as the aforementionedManzolini reference, with the exceptions that the Manzolini referencedid not describe recycle of exhaust gas or water gas shift reactors. Useof the anode output as a feed for the combustor resulted in a predictedfuel cell area of ˜197 km² in order to reduce the CO₂ content of thecathode output to ˜1.5 vol %. The amount of CO₂ lost as part of thecathode exhaust was ˜147.5 lbs CO₂/MWhr. Due to the large fuel cell arearequired for capturing the CO₂, the net power generated was ˜609 MW perhour. Based on these values, the amount of fuel cell area needed tocapture a fixed amount of CO₂ could be calculated, such as an area offuel cell needed to capture a megaton of CO₂ during a year of operation.For Configuration 1b, the area of fuel cell required to capture amegaton was ˜107.6 km²*year/Mton-CO₂. The efficiency for generation ofelectrical power relative to the energy content of all fuel used in thepower generation system was ˜52.1%. Base case 1b may be compared to basecase 1a to show a result of adding water gas shift reactors. Base case1b may be compared to base case 0 to show a result of adding water gasshift reactors and ˜35% exhaust gas recycle.

In a second set of configurations (2a-2e), the anode output was recycledto the anode input. Configuration 2a represented a basic recycle of theanode output after water and carbon dioxide separation to the anodeinput. Configuration 2b included a water gas shift reaction zone priorto the carbon dioxide separation stages. Configuration 2c did notinclude a reforming stage prior to the anode input. Configuration 2dincluded a reforming stage, but was operated with a fuel utilization of˜50% instead of ˜75%. Configuration 2e was operated with a fuelutilization of ˜50% and did not have a reforming stage prior to theanode. Configuration 2g included a reforming stage and was similar toconfiguration 2b and 2d, but operated with a fuel utilization of ˜30%.

Three variations on the 2a configuration were simulated. The 2asimulation results shown in column 1310 were based on a configurationthat included EGR, while the simulation results shown in column 1312were based on a configuration that did not include EGR. The simulationresults shown in column 1312 were based on a configuration that did notinclude EGR and an operating voltage of about 0.65 was maintained. Inthe simulations of column 1310 and 1312 an operating voltage of about0.65 was maintained.

Recycling the anode output back to the anode input, as shown inConfiguration 2a, resulted in a reduction of the required fuel cell areaas compared to the relevant base case. In column 1310 the required fuelcell area was ˜174 km², in column 1312 the required fuel cell area was˜169 km², and in column 1310 the required fuel cell area was ˜131 km².As can be seen, the lowered voltage resulted in a lower fuel cell area.

The 2a configuration changed the CO₂ emissions from the cathode exhaust.In column 1310 the CO₂ emissions were ˜141 lbs CO₂/MWhr, in column 1312the CO₂ emissions were ˜217.9 lbs CO₂/MWhr, and in column 1314, the CO₂emissions were ˜141 lbs CO₂/MWhr.

In Configuration 2b, a water gas shift reaction zone was included toprocess that anode outlet flow prior to water and carbon dioxideremoval. Three variations on the 2b configuration were simulated. The 2bsimulation results shown in column 1316 were based on a configurationthat included EGR, while the simulation results shown in column 1318were based on a configuration that did not include EGR. The simulationresults shown in column 1320 were based on a configuration that did notinclude EGR and an operating voltage of about 0.65 was maintained. Inthe simulations of column 1316 and 1318 an operating voltage of about0.65 was maintained.

In Configuration 2b, the additional water gas shift reaction zoneincreased the hydrogen content delivered to the anode, which reduced theamount of fuel needed for the anode reaction. Including the water gasshift reaction zone in Configuration 2b resulted in a required fuel cellarea of ˜168 km² in column 1316, ˜164 km² in column 1318, and ˜129 km²in column 1320. The CO₂ loss from the cathode exhaust was ˜143 lbsCO₂/MWhr in column 1316, was 217.5 lbs CO₂/MWhr in column 1318, and was218.7 lbs CO₂/MWhr in column 1320. The area of fuel cell per megaton ofCO₂ captured was ˜101.1 km²*year/Mton-CO₂ in column 1316, was ˜114.9km²*year/Mton-CO₂ in column 1318, and was ˜90.1 km²*year/Mton-CO₂ incolumn 1320.

Configuration 2c can take further advantage of the hydrogen content inthe anode recycle by eliminating the reforming of fuel occurring priorto entering the anode. In Configuration 2c, reforming can still occurwithin the anode itself. However, in contrast to a conventional systemincorporating a separate reforming stage prior to entry into the fuelcell anode, Configuration 2c relied on the hydrogen content of therecycled anode gas to provide the minimum hydrogen content forsustaining the anode reaction. Because a separate reforming stage wasnot required, the heat energy was not consumed to maintain thetemperature of the reforming stage.

Four variations on the 2c configuration were simulated. The 2csimulation results shown in columns 1322 and 1324 were based on aconfiguration that included EGR, while the simulation results shown incolumns 1326 and 1328 were based on a configuration that did not includeEGR. The simulation results shown in columns 1322 and 1326 were based ona simulation where an operating voltage of about 0.70 was maintained.The simulation results shown in columns 1324 and 1328 were based on asimulation where an operating voltage of about 0.65 was maintained.

Configuration 2c resulted in a required fuel cell area of ˜161 km² forcolumn 1322, ˜126 km² for column 1324, ˜157 km² for column 1326, and˜126 km² for column 1328. The CO₂ loss from the cathode exhaust was142.5 lbs CO₂/MWhr for column 1322, 143.5 lbs CO₂/MWhr for column 1324,223.7 lbs CO₂/MWhr for column 1326, and 225.5 lbs CO₂/MWhr for column1328.

In Configuration 2d, reforming was still performed to convert ˜20% ofthe methane input to the anode into H₂ prior to entering the anode insimilar arraignment to configuration 2b. In contrast with 2b, the fuelutilization within the anode was reduced from ˜75% to ˜50%.

Four variations on the 2d configuration were simulated. The 2dsimulation results shown in columns 1330 and 1332 were based on aconfiguration that included EGR, while the simulation results shown incolumns 1334 and 1336 were based on a configuration that did not includeEGR. The simulation results shown in columns 1330 and 1334 were based ona simulation where an operating voltage of about 0.70 was maintained.The simulation results shown in columns 1332 and 1336 were based on asimulation where an operating voltage of about 0.65 was maintained.

Configuration 2e incorporated both the reduced fuel utilization of ˜50%of 2d as well as elimination of the reforming stage prior to the anodeinlet of 2c. Four variations on the 2e configuration were simulated. The2e simulation results shown in columns 1338 and 1340 were based on aconfiguration that included EGR, while the simulation results shown incolumns 1342 and 1344 were based on a configuration that did not includeEGR. The simulation results shown in columns 1338 and 1342 were based ona simulation where an operating voltage of about 0.70 was maintained.The simulation results shown in columns 1340 and 1344 were based on asimulation where an operating voltage of about 0.65 was maintained.

In Configuration 2g, reforming was still performed to convert ˜20% ofthe methane input to the anode into H₂ prior to entering the anode insimilar arraignment to configuration 2b and 2d. In contrast with 2b and2d, the fuel utilization within the anode was reduced from ˜75% or ˜50%to ˜30%.

Column 1350 describes results of a simulation performed with aconfiguration similar to the configuration shown in FIG. 9. In FIG. 9,the EGR 998 first goes through the cathode and then HRSG 854. A basecase simulation for this configuration was performed. The simulatedresults from the base case are shown in column 1309. In contrast to thebase case, the simulated results of column 1350 were based on a fuelutilization of ˜50% rather than ˜75%. In addition, the simulated resultsof column 1350 were based on a configuration where reforming was stillperformed to convert ˜20% of the methane input to the anode into H₂prior to entering the anode in a similar arraignment to configuration 2band 2d.

FIG. 14 shows results from simulations performed based on severalconfiguration variants and alternative operating conditions. Thesimulations of FIG. 14 took into account more factors than thesimulations explained previously with reference FIG. 11. Otherwise, thesimulations shown in FIG. 14 were similar to the simulations shown inFIG. 11, with a few variations added. For example, each case wassimulated at about 0.65 volts in addition to the about 0.7 volts used inthe FIG. 11 simulations. In addition, a case with 0% EGR was added toeach configuration. FIG. 14 shows configurations corresponding to a baseconfiguration as well as several configurations where a portion of theanode output was recycled to the combustion zone for the turbine. Unlessnoted, the exhaust gas recycle was ˜35% for the simulated results shownin FIG. 11. In FIG. 14, each configuration was run with either ˜35% or0% EGR as shown.

In addition to different configurations and alternative operatingconditions, FIG. 14 shows additional parameters that were not shown inFIG. 11. For example, FIG. 14 includes the approximate fuel utilization,approximate steam to carbon ratio, EGR %, whether or not water gas shiftreactors were present in the configuration to process the anode exhaust,the approximate internal reforming %, the approximate CO₂ concentrationin the cathode inlet, and the approximate O₂ content in the cathodeexhaust.

FIG. 14 shows simulation results for additional configurations thatincluded recycle of at least a portion of the anode exhaust to thecombustion zone for the turbine. In FIG. 14, Configuration 1b (column1404) was similar to Configuration 1a (column 1406), but also included awater-gas shift reaction stage prior to the CO₂ separation stages. Thus,Configuration 1b was representative of a conventional system, such asthe aforementioned Manzolini reference, with the exceptions that theManzolini reference did not describe a water-gas shift reaction stage orrecycle of exhaust gas. For the 1b configuration, the required fuel cellarea to achieve a CO₂ concentration in the cathode exhaust of ˜1.45% was˜197 km². The amount of CO₂ lost as part of the cathode exhaust was ˜147lbs CO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜107.6km²*year/Mton-CO₂. The total power generated was ˜609 MW. The electricalefficiency was ˜52.1%.

Configurations 3a, 3b, and 3d correspond to configurations where theanode output was used as an input for the combustion zone of theturbine. In these configurations, the H₂ content of the anode output wasavailable for use as a fuel in the turbine combustion zone. Thisappeared to be advantageous, as the carbon-containing fuel used togenerate the H₂ was generated in the anode recycle loop, where themajority of the resulting CO₂ can be removed via the cryogenicseparation stages. This could also result in a reduction of the amountof carbon containing fuel delivered to the combustion zone, but thereduction in carbon-containing fuel in the combustion zone could alsoresult in the reduction of the CO₂ concentration in the input to thecathode.

Configuration 3a was a configuration similar to Configuration 1a, butwith recycle of the anode exhaust to the combustion zone. Fourvariations on the 3a configuration were simulated. The 3a simulationresults shown in columns 1410 and 1412 were based on a configurationthat included EGR, while the simulation results shown in columns 1414and 1416 were based on a configuration that did not include EGR. Thesimulation results shown in columns 1410 and 1414 were based on asimulation where an operating voltage of about 0.70 was maintained. Thesimulation results shown in columns 1412 and 1416 were based on asimulation where an operating voltage of about 0.65 was maintained.

Column 1410 shows the simulated results produced from a configurationmost similar to the 1a base case shown in column 1406. The required fuelcell area to achieve a CO₂ concentration in the cathode exhaust of˜1.45% was ˜179 km². The amount of CO₂ lost as part of the cathodeexhaust was 150.4 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was −416.3 km²*year/Mton-CO₂. The total power generated was˜599 MW. The electrical efficiency was ˜55.5%. Relative to Configuration1a, Configuration 3a had a lower total amount of CO₂ captured (−1.88Mtons/year for Configuration 1a vs. ˜1.54 Mtons/year for Configuration3a). This was believed to be due to the reduced amount ofcarbon-containing fuel delivered to the combustion zone. However, thisalso appeared to result in a reduced CO₂ concentration delivered to thecathode input, which caused the model to show a reduced efficiency ofCO₂ removal for Configuration 3a. Configuration 3a appeared to haveseveral advantages relative to Configuration 1a. First, Configuration 3arequired a lower fuel cell area, so that the system in Configuration 3awould likely have a reduced cost. Additionally, the system inConfiguration 3a appeared to have improved electrical efficiency, whichcan indicate lower fuel usage, even after adjusting for the differentpower output of the configurations.

Configuration 3b was similar to Configuration 3a, but also included awater gas shift reaction zone prior to the cryogenic separation stages.Four variations on the 3b configuration were simulated. The 3bsimulation results shown in columns 1418 and 1420 were based on aconfiguration that included EGR, while the simulation results shown incolumns 1422 and 1424 were based on a configuration that did not includeEGR. The simulation results shown in columns 1418 and 1422 were based ona simulation where an operating voltage of about 0.70 was maintained.The simulation results shown in columns 1420 and 1424 were based on asimulation where an operating voltage of about 0.65 was maintained.

As with the simulations shown in FIG. 11, configuration 3b appeared tohave increased CO₂ emission via the cathode exhaust. This was believedto be due to the additional hydrogen delivered to the combustion zone,which can result in a corresponding reduction in the amount of CO₂ thecombustion exhaust used for the cathode input. However, the fuel cellarea was further reduced.

Configuration 3d was similar to Configuration 3b, but the anode fuelutilization was reduced from ˜75% to ˜50%. Four variations on the 3dconfiguration were simulated. The 3d simulation results shown in columns1426 and 1428 were based on a configuration that included EGR, while thesimulation results shown in columns 1430 and 1432 were based on aconfiguration that did not include EGR. The simulation results shown incolumns 1426 and 1430 were based on a simulation where an operatingvoltage of about 0.70 was maintained. The simulation results shown incolumns 1428 and 1432 were based on a simulation where an operatingvoltage of about 0.65 was maintained.

Configuration 3g was similar to Configuration 3b, but the anode fuelutilization was reduced from ˜75% to ˜30%.

Column 1436 describes results of a simulation performed with aconfiguration similar to the configuration shown in FIG. 9. In FIG. 9,the EGR 998 first goes through the cathode and then HRSG 854. A basecase 1a′ simulation for this configuration was performed. The simulatedresults from the base case 1a′ are shown in column 1309 in FIG. 13. Incontrast to the base case, the simulated results of column 1436 werebased on a fuel utilization of ˜50% rather than ˜75%. In addition, thesimulated results of column 1436 were based on a configuration wherereforming was still performed to convert ˜20% of the methane input tothe anode into H₂ prior to entering the anode in a similar arraignmentto configuration 3b and 3d.

FIG. 15 shows results from simulations performed based on severalconfiguration variants and alternative operating conditions. Thesimulations of FIG. 15 took into account more factors than thesimulations explained previously with reference FIG. 12. Configurations4d and 4e represent configurations where the remaining anode exhaustafter separation (removal) of CO₂ and H₂O was divided evenly betweenrecycle to the anode input and recycle to the combustion zone for theturbine. In order to provide sufficient hydrogen for both the anodeinput and the combustion zone, the anode fuel utilization inConfigurations 4d and 4e was set to ˜50%. Configurations 4d and 4e bothincluded a water gas shift reaction zone prior to the separation stages.Configuration 4d included a separate reforming stage for reforming ˜20%of the additional fuel input to the anode prior to the fuel entering theanode. Configuration 4e did not include a reforming stage prior to thefuel entering the anode input.

Four variations on the 4d configuration were simulated. The 4dsimulation results shown in columns 1510 and 1512 were based on aconfiguration that included EGR, while the simulation results shown incolumns 1514 and 1516 were based on a configuration that did not includeEGR. The simulation results shown in columns 1510 and 1514 were based ona simulation where an operating voltage of about 0.70 was maintained.The simulation results shown in columns 1512 and 1516 were based on asimulation where an operating voltage of about 0.65 was maintained.

Five variations on the 4e configuration were simulated. The 4esimulation results shown in columns 1520 and 1522 were based on aconfiguration that included about a 35% EGR, while the simulationresults shown in columns 1524 and 1526 were based on a configurationthat did not include EGR. The 4e simulation of column 1528 was aconfiguration that included about a 45% EGR. The simulation resultsshown in columns 1520, 1524, and 1528 were based on a simulation wherean operating voltage of about 0.70 was maintained. The simulationresults shown in columns 1522 and 1526 were based on a simulation wherean operating voltage of about 0.65 was maintained.

Configuration 4f was similar to Configuration 4e, with the exceptionthat the anode fuel utilization in Configuration 4f was ˜33%, as opposedto the ˜50% in Configuration 4e. Four variations on the 4e configurationwere simulated. The 4f simulation results shown in columns 1530 and 1532were based on a configuration that included about a 35% EGR, while thesimulation results shown in columns 1534 and 1536 were based on aconfiguration that did not include EGR. The simulation results shown incolumns 1530 and 1534 were based on a simulation where an operatingvoltage of about 0.70 was maintained. The simulation results shown incolumns 1532 and 1536 were based on a simulation where an operatingvoltage of about 0.65 was maintained.

Column 1538 describes results of a simulation performed with aconfiguration similar to the configuration shown in FIG. 9. In FIG. 9,the EGR 998 first goes through the cathode and then HRSG 854. A basecase 1a′ simulation for this configuration was performed. The simulatedresults from the base case 1a′ are shown in column 1309 in FIG. 13. Incontrast to the base case, the simulated results of column 1538 werebased on a fuel utilization of ˜50% rather than ˜75%. In addition, thesimulated results of column 1538 were based on a configuration wherereforming was still performed to convert ˜20% of the methane input tothe anode into H₂ prior to entering the anode in a similar arraignmentto configuration 4b and 4d.

Although the present invention has been described in terms of specificembodiments, it is not so limited. Suitable alterations/modificationsfor operation under specific conditions should be apparent to thoseskilled in the art. It is therefore intended that the following claimsbe interpreted as covering all such alterations/modifications as fallwithin the true spirit/scope of the invention.

What is claimed is:
 1. A method for capturing carbon dioxide from a combustion source, the method comprising: introducing a fuel stream and an O₂-containing stream into a combustion zone; performing a combustion reaction in the combustion zone to generate a combustion exhaust, the combustion exhaust comprising CO₂; processing a cathode inlet stream, the cathode inlet stream comprising at least a first portion of the combustion exhaust, with a fuel cell array of one or more molten carbonate fuel cells to form a cathode exhaust stream from at least one cathode outlet of the fuel cell array, the one or more molten carbonate fuel cells comprising one or more fuel cell anodes and one or more fuel cell cathodes, the one or more molten carbonate fuel cells being operatively connected to the combustion zone through at least one cathode inlet; reacting carbonate from the one or more fuel cell cathodes with H₂ within the one or more fuel cell anodes to produce electricity and an anode exhaust stream from at least one anode outlet of the fuel cell array, the anode exhaust steam comprising CO₂ and H₂; separating CO₂ from the anode exhaust stream in one or more separation stages to form a CO₂-depleted anode exhaust stream; passing at least a combustion-recycle portion of the CO₂-depleted anode exhaust stream to the combustion zone; and recycling at least an anode-recycle portion of the CO₂-depleted anode exhaust stream to the one or more fuel cell anodes.
 2. The method of claim 1, wherein a fuel utilization in the one or more fuel cell anodes is about 65% or less.
 3. The method of claim 2, wherein the fuel utilization in the one or more fuel cell anodes is about 30% to about 50%.
 4. The method of claim 2, wherein the one or more fuel cell anodes comprise a plurality of anode stages and the one or more fuel cell cathodes comprise a plurality of cathode stages, wherein a fuel utilization in a low utilization anode stage in the plurality of anode stages is about 65% or less, the low utilization anode stage corresponding to high CO₂-content cathode stage of the plurality of cathode stages, the high CO₂-content cathode stage having a CO₂ content at a cathode inlet as high as or higher than a CO₂ content at a cathode inlet of any other cathode stage of the plurality of cathode stages.
 5. The method of claim 4, wherein the fuel utilization in the low utilization anode stage is at least about 40%.
 6. The method of claim 4, wherein a fuel utilization in each anode stage of the plurality of anode stages is about 65% or less.
 7. The method of claim 1, wherein the combustion-recycle portion of the CO₂-depleted anode exhaust stream comprises at least about 25% of the CO₂-depleted anode exhaust stream, and wherein the anode-recycle portion of the CO₂-depleted anode exhaust stream comprises at least about 25% of the CO₂-depleted anode exhaust stream.
 8. The method of claim 7, further comprising passing carbon-containing fuel into the one or more fuel cell anodes.
 9. The method of claim 8, further comprising: reforming at least a portion of the carbon-containing fuel to generate H₂; and passing at least a portion of the generated H₂ into the one or more fuel cell anodes.
 10. The method of claim 8, wherein the carbon-containing fuel is passed into the one or more fuel cell anodes without passing the carbon-containing fuel into a reforming stage prior to entering the one or more fuel cell anodes.
 11. The method of claim 8, wherein the carbon-containing fuel comprises CH₄.
 12. The method of claim 1, wherein the combustion exhaust comprises about 10 vol % or less of CO₂, the combustion exhaust comprising CO₂ optionally comprising at least about 4 vol % of CO₂.
 13. The method of claim 1, further comprising recycling a second portion of the combustion exhaust to the combustion zone, the second portion of the combustion exhaust comprising CO₂.
 14. The method of claim 13, wherein recycling the second portion of the combustion exhaust to the combustion zone comprises: exchanging heat between a second portion of the combustion exhaust and an H₂O-containing stream to form steam; separating water from the second portion of the combustion exhaust to form an H₂O-depleted combustion exhaust stream; and passing at least a portion of the H₂O-depleted combustion exhaust into the combustion zone.
 15. The method of claim 13, wherein the second portion of the combustion exhaust comprises at least about 6 vol % CO₂.
 16. The method of claim 1, wherein the anode exhaust stream, prior to the separating CO₂ from the anode exhaust stream in one or more separation stages, comprises at least about 5.0 vol % of H₂.
 17. The method of claim 1, further comprising exposing the anode exhaust stream to a water gas shift catalyst to form a shifted anode exhaust stream prior to the separating CO₂ from the anode exhaust stream in one or more separation stages, a H₂ content of the shifted anode exhaust stream after exposure to the water gas shift catalyst being greater than a H₂ content of the anode exhaust stream prior to exposure to the water gas shift catalyst.
 18. The method of claim 1, wherein the combustion-recycle portion of the CO₂-depleted anode exhaust stream is combined with the fuel stream prior to passing the combustion-recycle portion of the CO₂-depleted anode exhaust stream to the combustion zone.
 19. The method of claim 1, wherein a cathode exhaust stream has a CO₂ content of about 2.0 vol % or less.
 20. The method of claim 1, wherein separating CO₂ from the anode exhaust stream in one or more separation stages comprises cooling the anode exhaust stream to form a condensed phase of CO₂.
 21. The method of claim 20, wherein separating CO₂ from the anode exhaust stream in one or more separation stages further comprises separating water from the anode exhaust stream prior to forming the condensed phase of CO₂. 